Computerized Topo Maps
How Good Are They?

Contour Interval & Scale

By Dr. Brad DeLong
Author of 4-Wheel Freedom: The Art of Off-Road Driving

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Contour lines. A topographic map contains contour lines that indicate the elevation of the terrain shown on the map. The closer the contour interval, the more detailed a map will be. Contour lines with a 20 foot altitude separation will show more detail than ones with 100 foot separation. DeLorme Topo USA 2.0 shows contour lines at 20 foot intervals. The USGS topo maps show them at 40 foot intervals, which is what they will be, then, in 3-D TopoQuads, Topo!, All Topo Maps, and Fugawi.

Scale. The scale on a map means the way distance on the ground is represented. A scale of 1:24,000, commonly used in USGS topo maps, means that one unit on the map represents 24,000 units on the ground. The units are commonly taken as being in inches. Therefore, one inch on the map represents 24,000 inches on the ground, which is exactly 2,000 feet, or 0.38 miles.

Since a foot contains 12 inches, and a mile contains 5,280 feet, the inches in a mile equals 12 X 5,280 = 63,360 inches. Therefore, a scale of 1:63,360 means that one inch on the map represents a mile on the ground. This is one scale used for United States Forest Service (USFS) maps, though 1:126,720 is also used, where one inch equals two miles.

Other common scales are 1:100,000 and 1:250,000, where one inch equals 1.58 and 3.95 miles, respectively. The scale of a paper map is indicated somewhere in the margin. On USGS topo maps, the scale is usually shown near the bottom margin, along with the interval of the contour lines.

On a paper map, what you see is what you get. If the paper map says the scale is 1:24,000, then you know if you measure one inch on the map, it will represent 2,000 feet or 0.38 miles. However, on the computer screen, the actual scale of the map depends on many variables, including the size and resolution settings of the computer monitor, the resolution at which the map was scanned, and the zoom settings of the map program.

Since four of these programs use scanned USGS topo maps, some of the maps will show the scale on the computer screen, outside the margins of the map itself, and the scale will indicate the map as showing a 1:24,000 scale. There is a also a bar scale toward the center of the bottom map border which is accurate as displayed on the screen. But the bar scale will probably not fit any ruler scale you have and it is difficult to try to make a measurement on the computer screen anyway. Most programs have a way to set two points with the mouse and display the actual ground distance between the points.

Computerized Topo Maps Home Page
Part I: Introduction | Basic GPS w/ Terms | Interfacing with GPS | Contour Interval & Scale | Detail & Quality
Part II: Introduction | Installation and Technical Support | Ease of Use | Entering Waypoints and Routes
Part III: Introduction | Downloading Tracks from your GPS | Actively Tracking from your GPS | Printing Maps
General Comparison of the Programs

Product and Ordering Information from 4X4BOOKS.com
Topo USA 2.0 | 3-D TopoQuads | Wildflower TOPO! and TOPO! GPS | TrailSmart | ALL TOPO MAPS | Fugawi

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