A marked point of known elevation used as reference in calculating elevations of nearby locations; usually fixed to a permanent structure or base.

Contour line
On topographic maps, a line which passes through points of equal elevation; provides a method of showing the shape of the ground surface.

Contour interval
On topographic maps, the difference of elevation represented by each contour line. The interval used is chosen based on the scale of the map and the local relief. Flat areas will have a lower interval, while mountainous areas will be mapped with a larger interval.

Degrees, Minutes, Seconds
A geographic measurement system used for mapping the Earth's surface. The globe is divided into 180 degrees of latitude (90 on each side of the Equator); and 360 degress of longitude (starting at the Prime Meridian and counting up to 180 degrees to both the East and West).

Each degree is divided into 60 minutes. Each second is divided into 60 seconds.

Parallel imaginary lines are drawn encircling the Earth parallel to the Equator. These lines are used to measure distances North and South. The Equator is 0 degrees latitude; Pittsburgh, Pennsylvania is near 40 degrees 26 minutes North latitude.

Imaginary lines are drawn encircling the Earth which come together at the North and South Poles. These lines, called meridians, are used to measure distances East and West. The Prime Meridian is 0 degrees longitude and passes through Greenwich, England; exactly opposite this line and running through the Pacific Ocean, the International Date Line roughly follows the meridian at 180 degrees. This is also the meridian at which the UTM grid begins. Pittsburgh, Pennsylvania is near 80 degrees West longitude.

Map projection
A method of representing the three-dimensional surface of the earth on a two-dimensional map.

A Mercator map projection tends to stretch features East-West in an effort to compensate for the converging meridians toward the poles, thereby allowing a rectangular map to be drawn. This also allows adjacent maps to be easily aligned for an apparent continuous image.

Normal Pool
The typical elevation of a lake or river, especially one controlled by a dam, which is between low water level and high water level.

North American Datum (NAD)
A coordinate reference system used for horizontal positioning. Many USGS topo quads are based on the North American Datum of 1927. There is a conversion underway to a revised data set of 1983 (NAD83). Both are indicated on some newer maps; often a conversion method is given for deriving NAD83 from 1927 NAD shown.

The 1927 NAD is defined by the latitude and longitude of an initial point (Meade's Ranch in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The new NAD83 is based on a newly defined spheroid (GRS80); it is an Earth-centered datum having no initial point or initial direction.

A vertical datum is a level surface to which elevations are referred. In the US, vertical positioning is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29), formerly called "mean sea level 1929," and the North American Vertical Datum of 1988 (NAVD 88).

Quadrangle (Quad)
A map which covers a four-sided area bounded by parallels of latitude and meridians of longitude. Although the map is drawn as a rectangle, the area it represents is not in fact rectangular, due to the curved surface of the Earth and the convergence of the meridians at the poles. On a USGS 7.5' topo quad, the area represented varies from about 49 square miles in the northern US to about 70 square miles in the southern US.

The general shape of the ground surface in describing the variations between the high and low points.

A ratio of the size of features appearing on a map or drawing compared to their actual size. On a USGS 7.5' topographic quadrangle, the scale is 1:24,000, which is one inch on the map equals 24,000 inches (2000 feet). On some maps in this series, the scale may be 1:20,000 or 1:25,000. These are known are large scale maps because the features are shown in a relatively large size.

A Small scale map shows more land area, but with smaller representations and, therefore, lesser detail. A 15-minute series map is drawn at a scale in which 1 inch on the map equals just under 1 mile (1:62,500).

Topographic (Topo)
A type of map which shows the physical features of the land including the contour, some general types of vegetation and water; may also shows man-made features such as political boundaries, roads, utilities and structures.

Univeral Transverse Mercator (UTM)
A map projection which is measured in meters. On USGS 7.5' topo quad map, each grid tick equals 1,000 meters. This grid system provides a precise and convenient way to locate features on the map.

In UTM, the earth is divided into 60 zones of longitude. The conterminous 48 states of the United States are covered by Zones 10-19.

For structures described in this website, UTM coordinates are given as follows: Braddock - Zone 17; 0604 4472 (1927 NAD)

This means "Braddock" is the name of the USGS 7.5' series topographic quadrangle map. All locations on this map are in UTM Zone 17. The east coordinate 0604 is shown 604 at the grid tick mark at the horizontal edge of the map. The north coordinate 4472 is shown 4472 at the grid tick mark at the vertical edge of the map.

United States Geological Survey (USGS)
An agency of the United States government, part of the US Department of the Interior, which compiles and publishes geophysical data; publisher of many types of maps, photos, and digital mapping data.

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Page created: 24-Feb-1999
Last modified: 24-Feb-1999