United States Search and Rescue Task Force
What is a Topographic Map?
A topographical map is a representation of the Earth, or part of it. Traditionally, maps have been printed on paper. When a printed map is scanned, the computer file that is created may be called a digital raster graphic.
The distinctive characteristic of a topographic map is that the shape of the Earth's surface is shown by contour lines. Contours are imaginary lines that join points of equal elevation on the surface of the land above or below a reference surface such as mean sea level. Contours make it possible to measure the height of mountains, depths of the ocean bottom, and steepness of slopes.
A topographic map shows more than contours. The map includes symbols that represent such features as streets, buildings, streams, and woods. These symbols are constantly refined to better relate to the features they represent, improve the appearance or readability of the map, or to reduce production cost.
Consequently, within the same series, maps may have slightly different symbols for the same feature. Examples of symbols that have changed include built-up areas, roads, intermittent drainage, and some type styles. On one type of large-scale topographic map, called provisional, some symbols and lettering are hand drawn.
|Part of a 7.5-minute topographic map at 1:24,000 scale|
A topographic map tells you where things are and how to get to them, whether you're hiking, biking, hunting, fishing, or just interested in the world around you. These maps describe the shape of the land. They define and locate natural and manmade features like woodlands, waterways, important buildings, and bridges. They show the distance between any two places, and they also show the direction from one point to another.
Distances and directions take a bit of figuring, but the topography and features of the land are easy to determine. The topography is shown by contours. These are imaginary lines that follow the ground surface at a constant elevation; they are usually printed in brown, in two thicknesses. The heavier lines are called index contours, and they are usually marked with numbers that give the height in feet or meters. The contour interval, a set difference in elevation between the brown lines, varies from map to map; its value is given in the margin of each map. Contour lines that are close together represent steep slopes.
Natural and manmade features are represented by colored areas and by a set of standard symbols on all U.S. Geological Survey (USGS) topographic maps. Woodlands, for instance, are shown in a green tint; waterways, in blue. Buildings may be shown on the map as black squares or outlines. Recent changes in an area may be shown by a purple overprint. A road may be printed in red or black solid or dashed lines, depending on its size and surface. Below, is an easy reference listing:
- Black – man-made features such as roads, buildings, etc.
- Blue – water, lakes, rivers, streams, etc.
- Brown – contour lines
- Green – areas with substantial vegetation (could be forest, scrub, etc.)
- White – areas with little or no vegetation; white is also used to depict permanent snowfields and glaciers
- Red – major highways; boundaries of public land areas
- Purple – features added to the map since the original survey. These features are based on aerial photographs but have not been checked on land.
From Near to Far: Distance
Maps are made to scale; that is, there is a direct relationship, a ratio, between a unit of measurement on the map and the actual distance that same unit of measurement represents on the ground. If, for instance, 1 inch on the map represents 1 mile (which converts to 63,360 inches) on the ground, the map's scale is 1:63,360. Below is a listing of the scales at which some of the more popular USGS maps are compiled.
A convenient way of representing map distance is by the use of a graphic scale bar. Most USGS topographic maps have scale bars in the map margin that represents distances on the map in miles, feet, and kilometers. The table below shows the corresponding area of coverage for each scale and the linear distance that each scale represents in inches and centimeters.
|Puerto Rico 7.5 minute||1:20,000||1,667 feet||200 meters||71|
|7.5-minute||1:24,000||2,000 feet||240 meters||40 to 70|
|7.5- by 15-minute||1:25,000||2,083 feet||250 meters (about)||98 to 140|
|Alaska||1:63,360||1 mile||634 meters (about)||207 to 281|
|Intermediate||1:50,000||0.8 mile||500 meters (about)||County|
|Intermediate||1:100,000||1.6 mile||1 kilometer (about)||1,568 to 2,240|
|United States||1:250,000||4 miles||2. 5 kilometers (about)||4,580 to 8,669|
The background of this web page is an example of a topographical (topo) map as described above. Below are the map characteristics.
The map legend contains a number of important details. The figures below display a standard USGS map legend. In addition, a USGS map includes latitude and longitude as well as the names of the adjacent maps (depicted on the top, bottom, left side, right side and the four corners of the map). The major features on the map legend are show and labeled below.
- Map Name
- Year of Production and Revision
- General Location in State
- Next Adjacent Quadrangle Map
- Map Scale
- Distance Scale
- Contour Interval
- Magnetic Declination
- Latitude and Longitude
Contour lines are a method of depicting the 3-dimensional character of the terrain on a 2-dimensional map. Just like isobars in the atmosphere depict lines of equal atmospheric pressure, contour lines drawn on the map represent equal points of height above sea level.
On multi-colored maps, contour lines are generally represented in brown. The map legend will indicate the contour interval—the distance in feet (meters, etc.) between each contour line. There will be heavier contour lines every 4th> or 5th contour line that are labeled with the height above sea level.
Top View of Mountain showing contours
Drawn Contour Lines
- Steep slopes – contours are closely spaced
- Gentle slopes – contours are less closely spaced
- Valleys – contours form a V-shape pointing up the hill – these V's are always an indication of a drainage path which could also be a stream or river.
- Ridges – contours form a V-shape pointing down the hill
- Summits – contours forming circles
- Depressions – are indicated by circular contour with lines radiating to the center