Some Geographic Glossary

Aerial Photography (More >>)

Aerial photos are routinely used for topographic mapping, urban planning, geological and soils mapping, military reconnaissance, forest management, disaster assessment, and a broad range of agricultural applications. The National Aerial Photography Program (NAPP) is an interagency Federal effort coordinated by the U.S. Geological Survey, to conduct aerial photography of the United States on a regular basis. 

Aerial Survey (More >>)

Aerial survey is the study of the earth's surface using images taken from aircraft. Aerial survey is used in mapmaking, agriculture, environmental studies, and military operations. The science of making extremely precise measurements of height, area, distance, and volume from aerial images and using these measurements to create detailed physical maps is called photogrammetry.

Digital Elevation Models (DEM)

A digital elevation model (DEM) is a sampled array of elevations for a number of ground positions at regularly spaced intervals. These digital cartographic/geographic data files are produced by the USGS and are sold in 7.5-minute, 15-minute, 2-arc-second (also known as 30-minute), and 1-degree units.

Digital Orthophoto Quadrangles (DOQ) (More >>)

A digital orthophoto quadrangle (DOQ) is a computer-generated image of an aerial photograph. It has been orthorectified--altered so that it has the geometric properties of a map. This process also enables accurate measurements of ground distance on the photos-just as you get on maps. The standard DOQ produced by the USGS is a black-and-white (gray-scale), or color infrared, 1-meter ground resolution image, covering 3.75 minutes of latitude by 3.75 minutes of longitude.

Digital Raster Graphic (DRG) (More >>)

A digital raster graphic (DRG) is a digitally scanned image of a USGS topographic map. As with the paper topographic maps, DRG's can be used for hiking, camping, hunting, or other recreational activities. The full potential of a DRG is realized when it is combined with other digital map data, such as a digital orthophoto quadrangles (DOQ) or digital elevation model (DEM). The resulting hybrid product is excellent for collecting and revising digital map data.

Geographic Information Systems (GIS) (More >>)

A GIS is mapping software, combined with a database, that links information about the location of objects with information about their characteristics. Just as on a paper map, a digital map created by GIS represents features on the map such as cities, roads and natural features such as lakes and rivers. The information is shown only if the user chooses, and can include much more data than can be shown on a printed map.

International Date Line (More >>)

The International Date Line is an irregular line drawn on the map of the Pacific Ocean, near, and in many places coincident with, the 180th meridian. It marks the place where navigators change their date by one day on a transpacific voyage. East of the line it is one day earlier than to the west.

Latitude and Longitude (More >>)

Latitude and longitude are a system of geometrical coordinates used in designating the location of places on the surface of the earth. Latitude, which gives the location of a place north or south of the equator, is expressed by angular measurements ranging from 0 degrees at the equator to 90 degrees at the poles. Longitude, the location of a place east or west of a north-south line called the prime meridian, is measured in angles ranging from 0 degrees at the prime meridian to 180 degrees at the International Date Line.

Maps (More >>)

A Map is representation of a geographic area, usually a portion of the earth's surface, drawn or printed on a flat surface. In most instances a map is a diagrammatic rather than a pictorial representation of the terrain; it usually contains a number of generally accepted symbols, which indicate the various natural, artificial, or cultural, features of the area it covers.

Map Projections (More >>)

A flat map cannot accurately represent the rounded surface of the earth except for very small areas where the curvature is negligible. To show large portions of the earth's surface or to show areas of medium size with accuracy, the map must be drawn in such a way as to compromise among distortions of areas, distances, and direction.

National Elevation Data (NED) (More >>)

The National Elevation Dataset is a new data product in raster format, assembled by the U.S. Geological Survey, designed to provide National elevation data in a seamless form with a consistent datum, elevation unit, and projection.

Orthorectified

An image that has been orthorectified is one in which displacements caused by the camera lens and angle and terrain have been removed. An orthorectified image combines the image characteristics of a photograph with the geometric qualities of a map.

Photogrammetry

 

The science of taking measurements from photographs or other types of images to make physical maps, including topographic maps. Photogrammetrists generally use photographs taken by a special camera on an airplane. However, images taken from spacecraft may also be used. Distortions in the pictures are corrected using a stereoscopic device called a stereoplotter (see Stereoscope). The stereoplotter creates a three-dimensional image by combining overlapping pictures of the same terrain taken from two different angles. Contours, roads, and other features are then traced from the three-dimensional image to form a map base.

 

Terabyte

An information unit of one million million (one trillion) bytes, written as the number 1 followed by 12 zeros. In computing, which is based on the binary number system, tera has a literal meaning of 1,099,511,627,776, which is the power of 2 closest to one trillion.

Topographic Maps (More >>)

In addition to showing general locations and political boundaries, topographic maps depict the geology and special features of an area. The distinctive characteristic of a topographic map is that the shape of the Earth's surface is shown by contour lines, which are imaginary lines that join points of equal elevation on the surface of the land.

United States Geological Survey (USGS) (More >>)

The USGS is a research agency of the U.S. Department of the Interior, established in 1879. It is responsible for interpreting and mapping the geology, hydrology, and topography of the United States and its territories and for investigating and appraising mineral, energy, and water resources. The agency investigates such natural hazards as volcanoes, earthquakes, and landslides. It performs scientific research and publishes the results of its studies. The USGS provides the images and maps for the TerraServer-USA.

Universal Transverse Mercator Projection (UTM)

The Universal Transverse Mercator (UTM) system consists of 60 north-south zones, each zone measuring 60 wide in longitude. Each of these zones is numbered consecutively, starting with zone 1 (between 1800 and 1740 west longitude) and increasing eastward to zone 60 (between 1740 and 1800 east longitude).