Topographic Map

Topographic Mapping Basics

Topographic maps have a wide variety of uses. They are used for navigation by hikers, hunters, military personnel, and the general public. They are also used by policymakers and planners. Topographic maps are often used as a background for other data and as a base map for various data collection projects.

The details required on topographic maps typically result in larger-scale products. Scale refers to the ratio of map distance to ground distance. One centimeter on a 1:10,000 scale map, for example, represents 10,000 cm or 100 m on the ground. Popular scales employed by national mapping agencies are 1:50,000 or 1:25,000 (or 1:24,000 in countries where nonmetric measurement systems are still in common use).

When a topographic map indicates that a hill has an elevation of 241 m, what does that really mean? The height of 241 m must be measured from some location where the height is 0. In most cases, the elevation on a map is in reference to sea level. Tides and other factors make determining sea level complicated. As a result, numerous vertical reference systems, or datums, for sea level have been developed.

Topographic maps are typically projected into a rectilinear (Euclidean) horizontal coordinate system that allows measurements in linear units such as meters. The most common projection for topographic mapping is the Universal Transverse Mercator (UTM). Coordinate information is provided along the map edges and sometimes as an overlaid grid, referred to as the graticule. Much like height values, horizontal position is in relation to a datum. For example, the most common datum found on current topographic maps in the United States is the North American Datum 1983 (NAD 83).

Accuracy of elevation data on topographic maps is an important issue. Mapping standards for topographic maps are generally well specified and rigorously [Page 480]applied. In the United States, national map accuracy standards stipulate that 90% of all contours should be accurate to within one half of a contour interval. Ninety percent of all spot elevations should be accurate to within one fourth of a contour interval.

Creating Topographic Maps

Early topographic maps were laboriously made in the field with a technique called plane table surveying. A plane table is a drawing board with a sighting device. The mapmaker moves to various locations and draws visible features on the map. In the 1930s and 1940s, aerial photography and photogrammetry revolutionized topographic mapping. Mapmakers could now generate accurate topographic maps over large areas more efficiently than ever.

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