7.5-minute Quadrangle Maps

7.5-minute quads are the most commonly used maps published by the USGS. They cover 7.5 minutes of longitude and latitude and are available for all areas of the United States except for non-urban areas of Alaska. They have a scale of 1:24,000 and the area represented by each quad varies from about 64 sq. miles at 30° north to 49 sq. miles at 49° north. The 7.5-minute series employs Universal Transverse Mercator projection due to the importance of maps for navigation. Over 57,000 quads are used to cover the continental U.S., Hawaii, and portions of Alaska.

Aerial Photograph

An image of a portion of the surface of the earth that is either captured from an oblique angle or from a point perpendicular to the subject area, depending on the intended use. Aerial images are often orthorectified, the result of which is that every point on the map is geographically correct and the image can be rendered as a map.

Almanac

Data downloaded from the satellites that contains the identity codes, location, and time information for each satellite.

Altitude

The vertical distance of a point either on or above the surface of the earth from mean sea level. The term can also be used to measure the angle of a celestial body above the visible horizon.

Arctic Circle – Antarctic Circle

The Arctic Circle and its identical twin, the Antarctic Circle, are imaginary lines of latitude drawn at approximately 66° 33' north and south of the equator respectively. Resulting from the 23° 27' tilt in the earth’s axis, they represent the latitude at which, on the summer solstice, there is 24 hours of sunlight and on the winter solstice, there is 24 hours of darkness.

Aspect

The relationship between the surface distance represented by degrees of latitude and longitude. Moving north or south from the equator, where the aspect ratio is 1:1, the difference between the two increases. How this ratio is reflected on a map depends on the projection to be used.

Atlas

A bound collection of maps of a reasonably uniform nature.

It was Gerardus Mercator a 16th-century Flemish cartographer who first embellished the title page of his book of maps with an image of Atlas, the Greek God of the Earth, supporting the globe on his shoulders. The Book of Atlas became abbreviated to simply Atlas over the next few centuries.

Attribute

In GIS, information or data that applies to a specific geographic object, area, or location.

Atomic Clock

A type of clock that uses an atomic resonance frequency standard as its counter.

Average Speed field

When tracking with GPS, displays your average speed.

Axis

The earth’s axis is a 7,899.8-mile (12,713.5-km) long straight line from the North Pole through the Center of the earth to the South Pole. It is around this axis that the earth rotates once every 24 hours in a counterclockwise direction when viewed from above the North Pole. The axis is tilted at an angle of 23° 27’ in relation to the sun which results in seasonal variations in the sun’s intensity on different parts of the earth and thus results in seasonal variations in climate.

The direction of travel or the direction between two points in reference to true or magnetic north. When expressed in degrees, its value ranges from 0 to 360°. A compass heading is an azimuth.

In most places the word bearing has grown to mean the same thing as azimuth. However, azimuth is always measured from true or magnetic north in a clockwise direction. For example; due east is 90 and due west is 270 . See also, Bearing.

Base Map

A map designed specifically to be used as a template, onto which further information is placed for the purpose of comparison or spatial correlation. Base maps are often simple outline maps and are usually devoid of any information deemed unnecessary for basic geographical reference.

Bathymetry

The relief of ocean and lake floors. Bathymetry is usually measured in units of distance below mean sea or water level and is mapped using contour lines called isobaths in a similar fashion to terrestrial topographic maps.

Bearing and azimuth are essentially the same thing but the directional angle may be expressed in a different way. Whereas azimuths are measured clockwise from 360°, bearings can be described in terms of the quadrant in which they lie. For example, an azimuth of 150° could be South 30° East, meaning it is 30° east of south.

When tracking, bearing displays the direction of travel between your current position and your next waypoint, relative to true or magnetic north.

Like an azimuth, a bearing is measured in reference to true or magnetic north, but its value never goes over 90°. A bearing is always measured from north or south. A typical bearing would be N45 E, which is the same as an azimuth of 45°. The bearing S45 W is an azimuth of 225°. The use of the word bearing has changed over the years and now means the same as azimuth.

Benchmark

In the process of surveying an area, indicators are placed at significant locations to act as reference points for further measurements. These benchmarks usually give elevation based on a particular datum and may also detail latitude, longitude, and the date of the survey. The USGS uses metal discs about 4 inches in diameter embedded in concrete or attached in some way to a building.

Bluetooth

Bluetooth wireless technology is the low-power, short-range radio technology that allows electronic devices such as mobile phones, headsets, PDAs, notebook PCs and even cars to "talk" to each other without wires.

Buffer

In GIS, a polygon generated around an existing geographic object that is often used for proximity analysis.

Cadastral Map

Includes the length and direction of each side of a property boundary and the area of each parcel.

Cartesian Coordinate System

A system of quantifying location based on the relationship of an object to an X and Y axis where the surface of the earth is assumed to be flat. Measures of distance and direction are constant within a Cartesian system.

Cartography

The discipline of collecting, processing and communicating geographic information using either print or digital media.

Centroid

In GIS, the geometric or geographic center of a polygon.

Chart

A map that is designed to function primarily as a navigational aid. The most common of which are nautical and aeronautical charts. Because of the nature of their use, charts must be created with a high degree of accuracy and are frequently updated and revised.

Choropleth Map

A descriptive map that indicates, through some form of shading or pattern, the magnitude of a particular phenomenon within a defined geographical area. With any number of these areas on a given map an instant picture of the spatial pattern can be derived.

Circumference

Represented by a line around the earth that starts and finishes at the same point. Due to the fact that the earth is not a true sphere but an oblate spheroid, or geoid, its length varies from 24,901.55 miles (40,075 km) around the equator to 24,859.82 miles (40,008 km) around the poles.

Contour

An imaginary line on a topographic map connecting points of the same elevation above or below mean sea level, based on a particular datum. Usually these lines are drawn at a uniform interval throughout the map and every forth or fifth line is labeled, normally in meters or feet. For the map viewer they give a rudimentary impression of three-dimensionality.

Contours that appear relatively close together or that create a darker pattern, indicate that there is an abrupt change in elevation or a steep slope. Contours widely spaced depict relatively flat terrain. All contour lines are continuous with no beginning or end, they do not intersect or cross and only when showing truly vertical slopes do they appear to touch.

One of the most common misconceptions about contours is that an abundance of contours on a map means that the terrain is steeply sloped. Bearing in mind that each line simply represents the position of transition from one range of elevations to another, a high density of contours with the same label may simply indicate that the topography has a range of a few feet either side of the elevation indicated by that line. For all intents and purposes the terrain in this case is flat.

The need to accurately depict relief on maps has been a challenge to cartographers for millennia. In the 16th century Leonardo da Vinci developed an artistic system of depicting changes in elevation which coincided with advances in surveying techniques. Ironically contour lines were first used to represent bathymetry and were not applied to terrestrial landforms until late in the 19th century.

Contour Interval

The spacing of contours. There is no standard for how many contours should appear on a map and how they are spaced in terms of elevation. The decision is usually based on the nature of the relief and the scale. If the terrain is relatively flat, it is preferable to have more contours to show any minor variations. If the relief shows steep slopes or if the map is of a small scale, an abundance of contours creates clutter and makes it unreadable.

Coordinates

A coordinate system must have a point of origin and each location must be stated in terms of direction and distance from this point. Considering the globe, the usual point of origin is the equator and the prime meridian intersect (400 miles (643km) south of Accra, Ghana in the Gulf of Guinea). This point is 0° latitude and 0° longitude and it is from here that all positions of the globe are measured using the earth’s coordinate system. In this system, which closely resembles the Cartesian grid system used on a two-dimensional plane, the equator represents the X-axis and the prime meridian represents the Y-axis. However unlike a true Cartesian system there are definite limits to the extent of both axes. The X-axis terminates at 180° of longitude from both east and west and the Y-axis terminates at the North and South Poles. Thus all points on earth are between 0° and 90° north or south of the equator and 180° east or west of the prime meridian.

There are several variations in the global coordinate system usually based on recognized lines of latitude and longitude but with a different labeling system.

The Universal Transverse Mercator (UTM) coordinate system designates 6° longitudinal strips and divides these into 8° latitudinal zones. Coordinates are measured in actual distance in meters from the equator and in distance from the central meridian of this zone to the point in question. The zones themselves are assigned an alphanumeric code for quick regional locating.

The Military Grid Reference System (MGRS) takes each zone of the UTM system and further divides it into 100 km (62 mile) grid squares with each have an alphabetic code. Actual points within these zones are measured as easting and northing values originating from the southwest corner.

The World Geographic Reference System (GEOREF), primarily used for aircraft navigation, divides the world into 12 bands of latitude and 24 zones of longitude, each 15 degrees in extent. The resulting zones are given an alphabetic code and are further divided into one-degree squares with further alphabetic codes assigned.

Although such global systems can be easily adapted to relatively large-scale maps, more localized systems have been developed by individual countries and institutions to better suit their needs. Numerous national grids such as the British National Grid emerged to cover well-defined geographical areas and are often based on a local datum.

Course

The azimuth and length of a line, considered together.

Datum

A reference point or surface against which position measurements are made, and an associated model of the shape of the earth for computing positions.

Datums can be vertical, horizontal, or complete. Vertical datums deal primarily with elevation and are usually developed to supplement existing datums. Horizontal datums deal with assigning coordinates to positions on the surface of the earth based on a two-dimensional plane. Complete datums combine horizontal and vertical datums to provide a three-dimensional model. A datum can be developed to meet the needs of a specific region or country or can be global in scope in which case they are called geodetic datums.

The most commonly used datum today is the World Geodetic System 1984 or WGS84. This developed out of and is essentially the same as the North American Datum of 1983 or NAD83, which in turn was an update of the North American Datum of 1927 or NAD27. Many USGS maps, including the 7.5-minute quads, use NAD27 and the difference between it and NAD83 can be as much as 650 feet (200 meters). This is an important consideration when using a GPS receiver in conjunction with a paper map and many of the more recent versions of the USGS maps provide information on converting between datums.

The degree of variation between true north and magnetic north that varies with location and changes over time as the position of the magnetic north pole migrates.

Degrees

A degree is the most common unit used for defining a position on the surface of the earth. The polar and equatorial circumferences are divided into 360 degrees. Latitude is measured in degrees north or south of the equator with the North and South Pole being 90° north and south, respectively. Longitude is measured east and west from the prime meridian to 180° at the International Date Line.

Digital Elevation Model (DEM)

A file containing elevation points that have been sampled at regularly spaced intervals. A DEM is used for the generation of contours, shaded relief, 3-D terrain models and elevation profiles.

Differential GPS (DGPS)

A technique to improve GPS accuracy that uses pseudo-range errors recorded at a known location to improve the measurements made by other GPS receivers within the same general geographic area.

Dilution of Precision (DOP)

The total effect of all error sources in locating a GPS position.

Download

To transfer information from a remote unit, such as a GPS receiver, to a computer.

When the Cartesian coordinate system is used in conjunction with a grid system on a map, measurements along what would be the horizontal or X-axis are called eastings. These originate at the southwest corner of the map, a point known as the false origin. In grid coordinates, the easting is listed before the northing.

Elevation

Often used interchangeably with altitude, elevation refers to the vertical distance above or below mean sea level, of a point on the surface of the earth.

Ephemeris

Data which indicates the position and status of satellites.

Equator

The equator, at 0°, is the reference point for measuring positions of latitude. Halfway between the poles, it is the longest great circle route, stretching for 24,901.55 miles (40,075 km). It is the latitude at which the sun is directly overhead on the vernal and autumnal equinoxes.

Equinox

The year-long elliptical journey (revolution) of the earth around the sun, combined with the fact that the earth’s axis tilts in a consistent manner in relation to the sun, results in seasonal variations in the angle of the solar rays hitting the earth.

On the surface of the planet this gives the impression of the sun being lower in the sky in the winter months and higher in the summer.

There are two positions along the revolutionary path where the angle of the sun's rays is perpendicular to the axis of tilt. At these points the sun appears to be exactly overhead at the equator. These are the equinoxes, ("equal nights" in Latin), and they occur around March 20th (the vernal or spring equinox) and September 23rd (the autumnal equinox). Because the time duration of the earth's revolution is not a whole number of days, the date of the equinox can vary by up to two days from year to year.

A selected point on a map's grid system from which the coordinates of any point in the map area are measured. The false origin differs from the true origin in order to exclude negative values from the coordinates.

In GIS, a column containing a specific class of data for each object in the dataset.

Geocaching

An outdoor activity that most often involves the use of a Global Positioning System ("GPS") receiver or traditional navigational techniques to find a geocache (or "cache") placed anywhere in the world. A typical cache is a small, waterproof container containing a logbook and "treasure", usually trinkets of little value.

Geocode

The process of determining the precise geographic location or a set of coordinates for an object derived from non-geographic information such as a address or ZIP Code.

Geodesy

The science that is concerned with the measurement and mathematical description of the size and shape of the earth.

Geoid

Used to describe the exact shape of the planet, assuming all features, both bathymetric and topographic, are flattened to mean sea level. This shape is often referred to as an oblate spheroid.

Georeference

The process of geographically modifying a map or aerial photograph so that every point on the map is geographically correct.

GIS - Geographic Information System

A tool for analyzing and querying spatial data. Its function is facilitated by computer hardware, GIS software, and the technical expertise of the people involved.

Global Positioning System (GPS)

A system of transmitting satellites and ground control stations that allows someone with a suitable signal receiving and processing device (GPS receiver) to accurately determine their location at any point on or above the earth. GPS technology is used for accurate feature locating, object mapping, and point-to-point navigation.

Graticule

The pattern created by intersecting lines of latitude and longitude (parallels and meridians). This configuration makes several assumptions about the shape of the globe, most notably that it is a perfect sphere.

Great Circle

Any line representing the earth’s circumference is known as a great circle, the best known of which is the equator.

Grid

A network of uniformly spaced horizontal and vertical lines employed to allow an exact reference to any point on a map. A grid pattern does not necessarily correspond with recognized lines of latitude and longitude, especially when dealing with relatively large-scale maps. In order for the grid pattern to remain uniform, there may be some variation in the apparent direction of north as represented by the grid and the direction indicated by meridians in the map area. This introduces a third definition of north, grid north, which is distinct from both true north and magnetic north.

The use of latitude and longitude for locating points and determining direction on large-scale maps is considered fairly cumbersome. In these cases a true Cartesian system works better. This designates the southwest corner as the point of reference, called the false origin, and spaces the grid lines at uniform intervals moving north and east. The line represented by the bottom of the map corresponds with the X-axis and the line represented by right side of the map corresponds with the Y-axis. A grid reference is an alphabetic, alphanumeric or numeric sequence starting with the X-axis or easting followed by the Y-axis or northing.

Heading

The direction you are traveling at the moment.

Hemisphere

One of two half-spheres created when a sphere is divided into two equal parts, using a plane passing through its center. The equator bisects the earth and creates the Northern and Southern Hemispheres; the prime meridian and 180° of longitude combine to create the Eastern and Western Hemispheres.

International Date Line

Traveling across the International Date Line from west to east, time jumps ahead one whole day and traveling across from east to west, time goes back one whole day.

While often mistakenly referred to as the line of 180° of longitude, in fact the actual line, while following 180° for much of its course from pole to pole, deviates in places to avoid land areas, such as eastern Russia and New Zealand.

Unlike lines of latitude that are positioned relative to the equator, nature gave no direction for a reference point for lines of longitude.

The placement of degrees of longitude is simply the work of humankind. Prior to the International Meridian Conference of 1884, each country developed a coordinate system specific to that particular country or region. Measures of latitude were usually consistent among these systems but longitudes varied. Because travel between countries was becoming commonplace and a more universal system was needed, the conference resolved to adopt Greenwich just outside of London in England as the point through which the prime meridian would pass. Latitude would, from then on, be measured east and west from this point. Interestingly, not all counties in attendance favored the system and the French and Brazilian delegates abstained from the final vote.

Not only did this action affect the coordinate system used to numerically document positions on the earth, it created a universally accepted way of measuring time or specifically determining the beginning and end of a day. If the local time at Greenwich is midnight early Monday morning, at all points to the east it is later in the day on Monday and at all points to the west it is still Sunday. Knowing that 360° of rotation is equal to 24 hours of time, at a position 180° from Greenwich there is 12 hours difference and locally it is around midday. However at 179° east it is about midday on Monday and at 179° west it is midday on Sunday.

In line on a map connecting points at which the values within the data being mapped are equal.

Position north or south of the equator in degrees, minutes, and seconds.

Latitude is the system of determining location on the surface on the earth along a given north-south line. Measured in degrees north and south of the equator, it was first used by Greek geographers over 2000 years ago.

Because a change in position along a north-south line results in a change in the elevation of celestial bodies relative to the visible horizon, calculation of Latitude has always been fairly straightforward and the calculations of the Greeks were remarkably accurate within their known world. They devised a series of parallel lines originating at what they imagined would be the equator of the earth and ending at the poles. Testament to the ingenuity of this system is the fact the geographical coordinate system for measuring latitude has changed little in the time since.

More recently a numerical pattern was applied to these lines based on the Babylonian system of numeration. Given that a circle is divided into 360°, the portion of the circle represented from the equator to either one of the poles is a quarter of a circle, or 90°. This angle can be visualized if the equator is considered a two dimensional plane and a line is drawn from a given position on the surface intersecting this plane in the center of the earth. The angle created by the line represents the latitude of the position. Lines of Latitude are often referred to as Parallels.

Layer

In GIS, a dataset containing geographic objects with accompanying attributes that can be displayed or modified separately from other datasets.

Legend

A legend, sometimes known as a key, is the part of a map or atlas that depicts and explains the meanings of the cartographic symbols used.

Line

In GIS, an object defined by a series of connected points.

Log

A record of the speed, direction, and route of travel as obtained via GPS.

Logging

Recording the speed, direction, and route traveled using GPS.

Position east or west of the prime meridian in degrees, minutes, and seconds.

Longitude is the system for determining location on the surface on the earth along a given east-west line. It is measured in degrees east or west of the prime meridian that runs through Greenwich, near London in England. All lines of longitude, or meridians, converge at the north and south poles; thus the distance represented by degree of longitude is variable, depending on latitude.

The determination of position on the surface of the earth along a given east-west line was a relatively recent achievement in the field of geographical study. Unlike latitude, which was easily measured using natural phenomena and well-defined reference points, the basis for longitudinal measurement is absent from the natural world. For this reason, global explorers, aided by proven latitudinal navigation techniques, ventured south much earlier than then headed west. Early reckoning of the positions of the continental landmasses was extremely inaccurate to the extent that Columbus, influenced by the maps of the Greek geographer Claudius Ptolemy, believed that there was an easy westward route to Asia. He thought he had reached Asia when he inadvertently discovered the American continent and thus gave the island group on which he landed the name West Indies.

Because the only measurable distinction between points of longitude is based on time, it was only after accurate timepieces were developed that longitude was mastered.

While the plane of reference for latitude, the equator, maintains is significance for it’s entire 360° circumference, the reference point for longitude, the prime meridian, extends for only 180°, pole to pole. A two dimensional plane created by this line and passing through the center of the earth would be semicircular in shape. The angle created by an imaginary line drawn from a given position on the earth’s surface to the to the center of the earth where it meets this plane, represents the number of degrees of latitude of that position east or west of the prime meridian.

Magnetic North

A shifting geographic location toward which compass needles point. Declination is the distance, in degrees, that location varies from True North.

Frequent expeditions have documented the location of the Magnetic North Pole and have proven the theory that the pole is not stationary but appears to meander randomly around the Arctic region. The present location is around N 78.3°, W 104.0° and the pole is moving north-northwest at about 9 miles (15km) per year. Measurements have also indicated that the Magnetic North Pole experiences diurnal variations. Resulting from electromagnetic interference from the sun, these can result in a daily elliptical path of up to 50 miles (80km) in radius. The documented position is simply an average of this daily variation.

It was over 100 years before the scientific community realized that there was a difference between True North and Magnetic North. Calculations based on observations taken over the next several centuries concluded that the Magnetic Pole was somewhere in Northern Canada. This was confirmed during expeditions to the region in search of the elusive Northwest Passage.

The use of the magnetic compass to aid navigation became widespread in the fourteenth century when popular belief suggested that the needle pointed towards the North Pole or True North. Some suggested that it pointed to the Pole Star, others that there was a mountain of magnetite at the North Pole.

The magnetic field is generated by the dynamics of molten material deep within the earth’s mantle. It exists, not only along a two-dimensional plane represented by the earth’s surface, but perpendicular to this plane. Therefore. a magnetic compass pivoted on a horizontal axis will show variations in the angle of this field in respect to the earth. Moving closer to the Magnetic North Pole this angle becomes greater and at the pole itself it is 90° or perpendicular to the earth’s surface. At this point there is no horizontal magnetic field and a conventional compass does not function.

Furthermore there is evidence to suggest that over the history of our planet the polarity of the magnetic field may have reversed over 25 times. This fact adds fuel to the argument that there is no up or down when considering north and south. A globe or map can justifiably have south at the top.

It seems ironic that the basis for global navigation is something as erratic and volatile as earth’s magnetic field. Unlike the early explorers whose travels were directed by the predictable pattern of celestial bodies, we must first compensate for the difference in the bearing of True North and Magnetic North (magnetic declination) for that particular place and time. There are only two meridians along which True North and Magnetic North are the same, , or the meridian of the Magnetic North Pole’s current location and the opposing meridian, presently W 104° and E 76°. All other locations on the planet must factor the difference when using a compass.

Map

A map is a graphic representation of the physical features (natural, artificial, or both) of a part or the whole of the Earth's surface. Using some form of projection, a map transforms data from a three-dimensional object, the spherical earth, to a two-dimensional plane, usually a paper surface or a computer screen.

Mean Sea Level

To say that a certain feature is a particular numeric distance high naturally assumes that measurement originated at a fixed reference point. Sea level varies spatially, diurnally, and seasonally and even at a given point in space and time, wave action ensures that the point of origin is constantly in motion. Therefore an average needs to be measured and a tidal datum developed. Hourly sea level measurements have been collected over a 19-year period corresponding with the moon’s Metonic cycle. In this time frame, the phases of the moon go through one complete cycle to the point that they recur in the same order on the same diurnal pattern. Using 19 years as a measure of mean sea level guarantees every possible tidal variation can be taken into account. The tidal datum is constantly being revised as new data is acquired because of the possibility of more long-term changes in sea level associated with global temperature change. As mean sea level rises, elevations throughout the world are reduced.

Military Grid Reference System (MGRS)

A zone locating coordinate system developed from UTM in which each 6° UTM zone is divided into 8° of latitude. Each of these quadrangles is assigned a letter C through X and is further divided into 100-km squares represented by a two-letter code. Further division of these squares will result in greater accuracy and are represented by subsequent easting and northing values from 0 to 9.

Minutes

The common measure for determining a position on the surface of the earth is the degree, and each degree is further divided into 60 minutes. Thus there are 21,600 minutes in one complete circumference of the globe. One minute of latitude or longitude covers approximately 1.15 statute miles (1.85km) of surface distance and is equal to one nautical mile. Moving away from the equator this number decreases to the point where lines of longitude converge at the poles.

Northing

When the Cartesian coordinate system is used in conjunction with a grid system on a map, measurements along what would be the vertical or Y-axis are called northings. These originate at the southwest corner of the map, a point known as the false origin. In grid coordinates, the northing is listed after the easting.

Neatline

A neatline is the boundary line separating the body of a map from the margin. On quadrangle maps, these are the lines of latitude and longitude that define the extent of the map. On maps that utilize a Cartesian coordinate system, the X and Y-axes are neatlines.

Oblate Spheroid

It was Charles Darwin who first suggested that the earth was not a true sphere but, due to centrifugal force created by rotation, had expanded perpendicular to the axis of rotation.

The simple exercise of standing in a spot, spinning around while watching what happens to your arms, demonstrates this principal. As a consequence of earth's rotation, there is approximately 0.3% of difference between the equatorial and polar diameter, 7,926 miles (12,756 km) and 7,900 miles (12,714 km) respectively. The resulting flattened sphere is known as an oblate spheroid or geoid.

Orientation

The alignment of a map relative to the points of the compass. The term was originally used to describe the practice of aligning a map so that east faced up.

Orthophotograph

The physical properties of conventional photographic equipment are such that the images created are inherently deformed from the nature of the feature they are attempting to capture. Moving away from the center of an image the level of deformation increases, as the angle at which these portions of the image were observed by the camera was greatest. When dealing with aerial photographs, further distortions of the image result from variations in the terrain. Higher elevations are essentially closer to the camera and, unless they are in the center of the image, the camera exaggerates their position towards the outside of the photograph.

In order to make aerial photographs useful for photogrammetric analysis and mapping, it is usually necessary to geometrically alter the photograph so that it appears that the camera was over every point of the image area at the time the photograph was taken, thus ensuring minimal distortion. The resulting georeferenced image is called an orthophotograph and allows accurate measurement of azimuth, distance, and area.

Orthophotomap

Orthophotographs can be altered in a way that gives them a specified scale and orientation similar to conventional maps. Unto this medium, elements of cartographic symbolism can be applied, such as contours or boundaries, resulting in an orthophotomap. In many ways these are more useful than topographic maps as they combine only those cartographic representations that are deemed necessary, along with photographic detail with a degree of physical accuracy cannot be questioned.

Photogrammetry

The discipline of obtaining precise measurements from aerial images.

Planimetric Map

Perhaps the simplest of the common map styles, planimetric maps are similar in nature and scope to topographic maps but with all indications of terrain eliminated. The uncluttered appearance of planimetric maps makes them ideal base maps for thematic representations.

Plat

A plat is a specific type of map drawn to delineate land ownership and subdivision boundaries as determined by a local survey. The direction and length of each segment of the boundary line in question are indicated on the map.

Point

In GIS, an object lacking size and shape, defined as a specific set of coordinates.

Poles

The Poles are the true starting points for global measurements, as they represent either end of the earth's axis. The significance of the equator in our perception of the world and it's importance with respect to measuring latitude is only so because of it being half way between the North and South Poles.

Polygon

In GIS, an enclosed area defined by three or more line segments.

Port

A hardware interface used by a computer to communicate with an external device.

Prime Meridian

The meridian (line of longitude) passing through the Royal Greenwich Observatory, Greenwich, England; it is the meridian at which longitude is 0 degrees. It is often referred to as the Greenwich Meridian.

The decisions of why and where to locate the prime meridian were taken at the 1884 International Meridian Conference. The only help that the natural world gave in this decision was that the line must begin at one pole and end at the other. The why part of the question grew out of a need for standardization in the way the increasingly interdependent world measured latitude and longitude. Latitude had always been fairly consistent but longitude varied from country to country. The world needed a single universal system and thus it was decided that a line passing through the grounds of the Royal Observatory in Greenwich, east of London, England would be used. From thence forth all points lying to the east but not past 180° would be considered in the Eastern Hemisphere and be measured in degrees east of Greenwich, while those to the west but not past 180° would be in the Western Hemisphere and be measured in degrees west of Greenwich. By this single act of a few individuals, the reference point for documenting longitude came into being.

For as long as cartographers have been making maps they have toiled with the problem of portraying the surface of a spherical object unto a flat plane. This process, without exception, leads to distortion in one or more of the following: direction, scale, distance and area. Some projections were developed to eliminate certain distortions while they exaggerated others, while some were developed to minimize all. Geometrically the perfect map projection is a globe, a three-dimensional object, however even that has limitations. It is difficult to produce, cumbersome to handle, awkward to use as a basis for area and direction measurement and less than half of the earth's surface can be viewed at one time.

There is no benchmark for cartographic projection, consequently there is no good or bad projection. A projection is only as good as its ability to convey the information for which it was developed or is used.

Public Land Survey System (PLSS)

A U.S. coordinate system, more commonly used in the western states, in which the land is divided in to a series of Townships and Sections that whose location are referenced to a north-south meridian and an east-west base line.

Query

In GIS, a set of commands that are used to select specific objects in a database.

Range

In the US Public Land Survey System (PLSS), a row of townships extending east and west. Also used to convey east-west location of a township relative to the survey's to the principle meridian.

Raster Map

A digital base map in which geographic objects are depicted using a grid of pixels, with the color of each pixel individually defined. Examples of raster maps include aerial imagery or scanned paper maps.

Real Time

The actual time during which something takes place.

Receiver

Hardware device that receives data, such as from satellites.

Remote Sensing

The practice of deriving information about the earth's surface from imagery or data captured above the surface.

Relief

Commonly used interchangeably with topography; however, they are not necessarily the same. Relief is the term used to specifically describe the pattern of landforms on the earth's surface, such as mountains, valleys, and bathyametric features.

Resolution

The level of detail that can be attained in a raster map, typically measured in distance on the ground per pixel on the map.

Rotation

The rotation of the earth, or the period of time needed to complete one spin on its axis, is 23 hours, 53 minutes, and 4 seconds. Rotation is measured against the sun whose position relative to the earth varies slightly each day. This gives us exactly 24 hours in each day.

Given that the circumference at the equator is 24,859.82 miles (40,008 km), the speed of rotation at this latitude is 1,036 mph (1670 km/h). To calculate the speed at other latitudes, use the following equation:

• Speed at Deg. of Lon. = Cosine of lat. x Speed at the equator.
• In Maine: Speed at Deg. of Lon. = Cos. of 45° x 1036 mph 733 = 0.7071 x 1036. In Maine the earth is rotating at 733 mph (1180 km/h).
• At 89° 50' north or 11.5 miles from the North Pole: Speed at Deg. of Lon. = Cos. of 89° 50' x 1036 mph 3mph=0.0029 x 1036 At a position 11.5 miles from either of the poles the earth is rotating at 3mph (5km/h). This would make it possible for someone to walk around the earth with the sun remaining in the same relative position.

Revolution

The earth's revolution is its elliptical path around the sun, the main tangible result of which is our seasonal variation in climate.

Owing to the consistency of the earth's 23° 27' angle of tilt, the sun's rays are perpendicular to the surface of the planet at different latitudes throughout the orbital journey. The range of these latitudes is from 23° 27' north of the equator (the Tropic of Cancer) to 23° 27' south of the equator (the Tropic of Capricorn). The former represents the position on the earth's orbit of the sun, at which the Northern Hemisphere is tilted towards the sun and thus coincides with summer in the northern latitudes. When the sun's rays are perpendicular to the latter the Southern Hemisphere is tilted towards the sun and the seasons are reversed.

This entire cycle takes a solar year or 365.25 days. In order to maintain the correct annual position in respect to the sun, the 0.25-day is accumulated for 4 years and the resulting whole day is then added at the end of February.

The difference in the major and minor radii of the orbital path is about 1.7% ranging from 91,400,000 miles (147,060,000 km) to 94,500,000 miles (152,050,000 km). The shortest radius coincides with summer in the Southern Hemisphere make it slightly warmer than summer in the Northern Hemisphere.

The journey of the earth around the sun covers a distance of approximately 583,400,000 miles (938,700,000 km) with an orbital speed of 66,000 mph (106,000 km/h).

Rhumb Line

Although the most direct route between two points on the surface of the earth is a great circle route, early maritime explorers were quick to ignore this logic due to navigational difficulties. As one follows a great circle, the bearing is not consistent and requires accurate mathematical computations to ensure that the desired destination is reached. Mercator's projection was developed to aid the early navigators as, although it ignored such cartographic details as displaying land area correctly, it maintained the correct bearing throughout the map. On a globe, all lines of longitude are perpendicular to all lines of latitude and it was this fact that Mercator wanted to display in his projection. Thus a straight line drawn on the map intersects all lines of longitude and latitude at a constant angle and represents a constant bearing. This is a Rhumb line or loxodrome and, although not showing the fastest route to a destination, is the direction taken by many maritime travelers.

Rubber Sheeting

The process of stretching or transforming the geometric properties of a raster base map for the purpose of aligning the map relative to the surface of the earth that it depicts. This is usually achieved by embedding known coordinates at several recognized locations in the raster map.

Satellite Image

An image of a portion of the surface of the earth that is captured by image sensors onboard an orbiting satellite and subsequently transferred to a control station on the ground.

Scale

An important property of a map is the scale. It can be indicated by a scale bar and/or a ratio 1:n. This enables the map user to measure a distance on the map and determine the distance on the ground.

The scale of a map is the relationship between the extent of features in the real world and the size at which they are portrayed on the map. Because of the problems inherent in transferring information from a spherical object to a flat plane, scale is usually variable on any given map. This variation can be between one position on the map and another, or between different directions originating at the same point. When dealing with larger scale maps of local areas, these differences are usually insignificant.

Scale can be expressed in several ways:

• A verbal representation, such as one-inch equals one-mile, may be used. Maps with a scale expressed in this way are often developed in such a way that the scale can be easily represented by primary units of a widely used system of measurement.
• Scale may be designated by a representative fraction (RF) or ratio, 1:1,000,000 or 1/1,000,000 for example. In this case, a single unit of measurement on the map is equal to 1,000,000 of those units on the ground. This type of scale lends itself well to metric measurements due to the relative ease of conversion between centimeters, meters, and kilometers. The same scale used with inches and miles would be 1 inch equals 1,000,000 inches, 27,780 yards or 15.78 miles on the ground. This is the most common method of scale representation and is often used as the descriptor for a series of maps. For instance the USGS produces what is commonly called the 1:100,000 series.
• A more visual and less mathematical expression of scale is a linear graph or bar that is divided into units of actual distance on the portion of the earth represented by the map. A portion of the graph may be subdivided into smaller units to allow a more precise estimation of distance on the map.

The terms "large" and "small" scale have been a constant source of much confusion for map users. Large in this case refers to the ratio or fraction of the scale. In other words a scale of 1:24,000 or 1/24,000 is larger than a scale of 1:100,000 or 1/100,000. The confusion stems from the fact that in this case 24,000 is larger than 100,000 and from the fact that a map of a smaller scale usually represents a larger land area.

Section

In the US Public Land Survey System (PLSS), a an area measuring roughly one mile by one mile that was originally the basic unit for land ownership.

Shaded Relief

A cartographic technique that provides an apparent three-dimensional rendering of the terrain, on maps and charts, by the use of graded shadows that would be cast by high ground if light were shining from the northwest.

Shapefile

A propriety spatial data file format that was developed by ESRI for use within its GIS software and which is now supported by virtually all GIS applications, including XMap/GIS Editor. A Shapefile is used to store information about the location of objects as well as information or attribute data that relates to each object. Although the term Shapefile is referred to in its singular form, a Shapefile is actually a set of up to eleven individual files, three of which are required when working with data in this format: *.shp, *.dbf, and *.shx. The *.shp file stores the geometric properties of the data; the *.dbf contains the attribute information about each object; the *.shx stores the index of the geometry.

Solstice

The word solstice comes from the Latin for sun stop, and describes the biannual event when the sun appears to stop it's latitudinal migration and move in the opposite direction. On or around June 21st, the summer solstice in the Northern Hemisphere, the earth is at the point on its revolutionary orbit where it is tilted directly towards the sun. The specific time varies because the revolution is not a whole number of days. At the time of the summer solstice, the sun appears to be directly overhead at the latitude of 23° 27' north of the equator, also known as the Tropic of Cancer.

The winter solstice, on or around December 21st, represents the point at which the earth's axis is tilted away from the sun. On this date, the sun's rays hit the earth perpendicular to the latitude 23° 27' south of the equator, the Tropic of Capricorn.

Spatial Data

Information that is attributed to a specific location, line, or area that is the key component of a GIS.

Spatial Query

In GIS, a data search function that returns records or objects that match user-defined geographic parameters.

Spline

A liner feature in which angles or changes in direction have been smoothed.

State Plane Coordinate System (SPCS)

A point locating coordinate system covering the entire US at a constant scale using a standard datum. The country is divided into 125 zones based on state boundaries and, if further division is necessary, county boundaries. For states with a natural north-south tendency, the grid is superimposed on a Transverse Mercator projection. For those running east west Lambert Conformal is used. Each zone has its own central meridian and false origin to the south and west of the zone. Coordinates are represented by the false easting and false northing measured in feet, followed by the state and zone.

Standard Meridian/Parallel

The line of control on a map projection; it is around this line that the alignment and scale of the map is determined. On most projections, distortion increases further from the standard line. Along the standard meridian or parallel, the two-dimensional plane of the projection intersects the globe.

Structured Query Language (SQL)

A computer language for creating editing and retrieving data from a relational database, commonly used in the management of a GIS.

Surveying

The discipline of measuring and recording geographic data.

Topographic Maps

Topographic maps are drawn in such a way as to show the natural landforms and man-made features (the topography) of a portion of the surface of the earth and are thus distinct from planimetric maps. Considered to be the most complete type of map, they are usually of a fairly large scale and represent both vertical and horizontal features in a measurable form.

Topography

Topography is often mistakenly used as an alternative to relief but the two are somewhat different. Topography refers to the description or representation on a map of all features in a given area, both natural and man made.

Topologically Integrated Geographic Encoding and Referencing (TIGER)

A spatial data file format developed by the US Census Bureau prior to the 1990 census. TIGER files are the foundation of DeLorme's road and street database as seen in products such as Street Atlas USA and Topo USA.

Topology

In GIS, the geometric characteristics of a vector object and the relationship between the nodes that define the size and shape of the object.

Township

In the US Public Land Survey System (PLSS), a an area comprised of 36 sections or 36 square miles. Also used to convey north-south location of the township relative to the survey's base line.

Track

To observe or plot the path of a moving object.

Triangulation

A land surveying process, using simple trigonometry, in which the length of a line between two points and the angle or bearing from these two points to a third point is used to calculate the distance to that point.

True North

True North is the direction of the North Pole from any point on the surface of the earth and is the orientation of all lines of longitude or meridians. It is not the direction indicated by the orientation of a compass, which is referred to as Magnetic North.

U.S. National Grid (USNG)

A zone locating coordinate system that uses an alpha-numeric characters to label zones derived from the Universal Transverse Mercator coordinate system. Developed by the US Department of Homeland Security, it is hoped that the USNG will provide an interoperable environment for location-based services by offering a consistent nationwide grid system.

Upload

To transfer information from a computer to a remote unit, such as a GPS receiver.

Universal Polar Stereographic (UPS)

A point locating coordinate system used to complete the global coverage of the Universal Transverse Mercator coordinate system by assigning a rectangular grid to the polar areas (north of 84° N and south of 80°).

Universal Transverse Mercator (UTM)

A point locating coordinate system based on the Transverse Mercator projection in which the world is divided into 60 zones of 6° longitude. Each zone stretches from the latitudes of 84° north to 80° south and these are labeled 1-60. UTM zones have an origin on the equator at the point where the central meridian of the zone intersects. Coordinates are measured in meters from the false origin followed by the zone number and the hemisphere. The false origin is 500,000 meters west of the central meridian, at the equator in the Northern Hemisphere and 10,000,000 meters south of the equator in the Southern Hemisphere.

Vector Map

A Base map in which geographic objects are stored in a database with each object having its own distinct characteristics, including its location, dimensions, object type, color, etc. The process of displaying a vector map for a particular area involves the querying of the database to select the objects that fall within the geographic area in question and displaying them appropriately on a map. In contrast to raster maps, vector maps are interactive in that objects can be individually modified which allows that map data to be periodically updated.

Waypoints

Marked positions with specific coordinates that can be downloaded or uploaded.

Wide Area Augmentation System (WAAS)

A program developed by the Federal Aviation Administration to enhance the accuracy of GPS receivers, usually to within three meters of actual position, by providing a modified GPS signal. After calculating the error factor at ground control stations through the US, the corrected signal is transmitted to two geostationary satellites that in turn transmit the GPS signal back to earth. In order to use the WAAS signal, a suitably equipped receiver, such as DeLorme Earthmate LT-20, must be used.

World Geographic Reference System (GEOREF)

A zone coordinate system that divides the world into 15° of latitude and longitude. Each of the resulting quadrangles is assigned a two-letter identifier. These are further divided into 1° by 1° areas and assigned a further two-letter code. If further precision is needed the number of minutes can be added to the coordinate value.

3-D TopoQuads

A DeLorme software product that offers seamless coverage of the USGS 7.5-minute series topographic maps for individual states, or in some cases, sub-state regions. 3-DTQ also refers to the same USGS base map dataset for use in XMap and the component of an Aerial Data Packet (ADP) that shows the 7½-minute series maps.

Add

When placing stops or vias along a route, selecting the Add option will place each successive point at the end of the stops and vias list, just before the finish.

Aerial Data Pack

A custom base map option for users of Topo USA and XMap that provides 10-meter resolution satellite imagery, 1-meter resolution aerial imagery, and scanned USGS topographic maps derived from the 1:24,000 series. The area of coverage is selected by the user from a grid that appears on the map after the Datasets button is selected within the Net link tab. The details are subsequently transmitted to DeLorme using the NetLink functionality after which the ADP can be downloaded or burned to CD or USB hard drive which is shipped to the user.

Attach

As in attaching a draw road to existing roads. See also, Snapping.

Base Data

In XMap, the DeLorme-provided map displayed in either the Primary or Secondary map window. A Base Maps can be either raster- or vector-based and include SAT 10, USA Topographic and XMap USA Street Network.

Blue Logger

DeLorme's GPS receiver and data collector that uses Bluetooth wireless technology to communicate with a host computer. Blue Logger offers the dual functions of real-time location tracking when used with DeLorme mapping software on either a PC or PDA, and standalone data collection based on parameters established using the Blue Logger manager software.

Bread crumb trail

A set of dots and arrows that display on your computer screen to record your progress as you travel.

Category

A search option within the Find tab that allows a Delorme software user to specify, by typing the appropriate word, the type of feature that is being sought.

Classify

A function of the GIS Editor tab in XMap/GIS Editor in which the visual characteristics of an Openspace GIS layer are modified to reflect a pattern or grouping of objects in a particular attribute field. 

Control Panel

In DeLorme software, the array of tools located to the right of the map window.

Data Zoom

The value used to represent the relative scale of the map view in DeLorme software. Increasing this value will result in larger scale map covering a smaller geographic area. When viewing a vector base map, adjusting the Data Zoom level will also change the visual characteristics and concentration of map data.

Date Field

When connected to a DeLorme GPS receiver, displays the current date.

DeLorme Raw File

A proprietary GPS data file format that can be collected using the Earthmate Blue Logger GPS receiver. GPS data in DeLorme Raw File format is used by GPS Post Pro 2.0 to calculate sub-meter accurate positional coordinates.

Dist to Finish

When GPS tracking, displays the distance from your current position to your Finish.

Dist to Next Turn

When GPS tracking, displays the distance from your current position to your next turn or route change.

Draw Layer

A DeLorme file used in all DeLorme software products that contains user-created annotation features that have been generated with the Draw tab tools.

Eartha

A 42 ft diameter rotating globe housed in the lobby of DeLorme headquarters in Yarmouth, Maine. Eartha displays enhanced satellite imagery and detailed bathymetry for the entire planet at a scale of 1:1,000,000.

Earthmate

The brand name for DeLorme's range of GPS receivers that provide tracking and in-vehicle navigation functionality when used with DeLorme software. Current models include the Earthmate LT-20 and Earthmate Blue Logger.

Elevation field (GPS)

When GPS tracking, displays your current elevation (due to Selective Availability which can vary as much as 512 feet (156 meters); however, 95 percent of the time you can expect greater accuracy.) Elevation is the height above sea-level.

Exchange

The function of DeLorme software that allows maps or user-added features to be uploaded to or in certain circumstances, downloaded from a handheld GPS receiver or a PDA on which Street Atlas USA 2006 Handheld Edition has been installed.

GeoID

In XMap/GIS Editor, a sequential value attributed to each object that is imported or created using the GIS Editor tools and that is displayed as the first field in the attributes table.

GeoSpider

A DeLorme software product offering surveyors and engineers tools for verifying the accuracy of terrestrial and GPS networks.

GPS Post Pro 2.0

A DeLorme software product that is used to process field-collected GPS location data and provides coordinates that are accurate to within a meter of their actual location.

ImageReg

A function of XMap in which a image file, such as an aerial photograph or scanned map, can be registered or aligned with the underlying base map. Also the name of the tab the contains the image registration functions.

Initialize

To set to a starting position, as in obtaining initial coordinates for a GPS receiver.

Insert

When placing stops or vias along a route, selecting the Insert option will allow the software to determine the optimal sequencing of these points that will result in the creation of the shortest or quickest route between the stop and finish.

LT-20

The latest generation of DeLorme's Earthmate GPS receiver that connects using a USB cable and provides tracking and in-vehicle navigation functionality when used with DeLorme software.

Map Data

In XMap, 3-DTQ and Topo USA, the tab containing the file and map management tools that allows the user to create, edit, save and open Projects.

Map File

In the Street Atlas product suite, a user-created file that saves the current map view as well as any objects that have been added to the map such as routes and draw layers. Also the name of the tab the contains the file management functions.

MapNote

A user-created draw object comprised of a text box with a pointer typically used to add  annotation about a particular location on the map.

Mobile Map Colors

A display option in DeLorme software that darkens the map colors thus reducing the brightness of the screen for in-vehicle use after dark.

Maximum Speed Field

When GPS tracking, displays your maximum speed.

NetLink

An embedded Web browser contained in tab that is included in all DeLorme software titles. NetLink offers the user a direct line of communication to DeLorme for product information and support and allows DeLorme to directly convey information on such issues as available product upgrades and enhancements. In XMap and Topo USA, NetLink can also be used to purchase Aerial Data Packs.

Openspace

DeLorme's GIS file format. Data layers imported into the GIS Editor tab in other spatial file formats are converted into Openspace format for use within XMap. When transferring GIS data layers to other XMap users, the use of the Openspace Transfer File allows the integrity of the data, including any visual characteristics, to be retained.

Overview Map

Found in the lower right corner of the screen, a map that displays a wider perspective of the current map view.

Photoflight

An advanced software product that provides tools for planning aerial photography missions. Built on the XMap platform, PhotoFlight automatically generates flight lines and camera trigger points that can be exported in one of several commonly used file formats.

Places of Interest (POI)

An accurately placed, searchable dataset of points incorporated into the Street Atlas USA base map, which includes over 4.2  million businesses, hospitals, schools, government offices, attractions and other services.

Primary Map

In XMap, the map on the right of the screen that is displayed when the Split Window toggle button is clicked.

Project

In XMap and Topo USA, a user-created file that saves the current map view as well as any objects that have been added to the map such as routes and draw layers.

Radar

A search function accessed using the Find tab that scans a user-defined area within the proximity of the current location for travel-related services.

Reference Data

In XMap, the core base map dataset that includes low-resolution map features that are visible from Data Zoom level 1.0 to 5.7. The Reference Data also contains and controls the display of other map components such as the map grid, USGS Quad coverage polygons, and elevation features.

Route

A user-created linear feature that provides optimal directions between a starting and finishing point. Depending on the DeLorme software title being used, routes may be created on roads, trails, or using direct or point-to-point calculation. Also the tab in DeLorme software that contains the route creation and management tools.

SAT 10

A optional base map dataset for use with XMap that displays 10-meter resolution, color, satellite imagery. Available for purchase by state or sub-state region.

Secondary Map

In XMap, the map on the left of the screen that is displayed when the Split Window toggle button is clicked. This window can be used to show a alternative base map dataset than that displayed in the Primary Map; to show a 3-D map; or to provide a close-up or wide area view of the current location.

Attaching a point on one draw object to the exact coordinates of a point in another draw object. You can snap the central shape point of an arc to another object or snap a routable road to an existing road, enabling routing from the drawn road to the road system on the map database.

Speed Field

When GPS tracking, displays your speed as you travel.

Stop

A user-selected location indicting a stopping point along a route.

Subtab

A button within a tab that reveals additional tools.

Symbolize

A function of the GIS Editor tab in XMap/GIS Editor in which the visual characteristics (such as color, opacity, line weight, or symbol style) of the objects in an Openspace GIS layer or an are manually modified.

Tab

In DeLorme software, one of several selectable functions found below the map window that provide the critical mapping tools of the program

Tab Manager

A feature of DeLorme software that allows the user to customize the display of the tabs by disabling those that are unused or unnecessary or by changing their order to suit personal preference. The Tab Manager is accessed using the Help menu at the top of the screen.

Time to Finish

When GPS tracking, displays the time from your current position to your Finish (based on your current speed).

Time to Next Turn

When tracking, displays the time from your current position to your next turn or route change (based on your current speed).

Tool Bar

In DeLorme software, the array of tools located along the top of the map window.

TopoBird

A wholly owned subsidiary of DeLorme that offers aerial mapping services using the latest in airborne technology. TopoBird provides high-resolution multispectral imagery and extremely accurate elevation data using an aircraft-mounted multi-spectral image sensor working in tandem with a LIDAR scanner.

Transfer File

A compressed file containing all of the contents of a Map File or Project. Allows the current map view as well as user-added features, such as Draw objects and routes, to be quickly and easily shared with other DeLorme software users. Transfer files are created within the Map Data or Map Files tab.