A tract’s terrain can determine the range of potential land management activities, sometimes inhibiting completely, other times making more costly, meaning landowners (and prospective land buyers) should know how to interpret maps with topographic data. Topographic maps depict three-dimensional elevation data in a two-dimensional space.
Topographic maps convey elevation data through contour lines. A contour line represents a single vertical elevation above mean sea level. Since each contour line depicts a single elevation, they never cross. If you were to pick a contour line and follow it, you would eventually complete a loop and end up at your starting point. Topographic maps have both thick and thin contour lines. The thick lines are called index lines and are labeled with the elevation. The thin lines are called contour intervals and are the intermediate elevation lines that fall between the index lines.
Reading Contour Lines
Viewing Figure 1, you’ll notice that the index lines are at the 700 and 800 feet elevations. Counting the contour lines between them, you can extrapolate that this map has a contour interval of 20 feet. You’ll also notice an “x” labeled 812 feet. Some hill and mountain summits will be marked with the actual elevation, denoted by an “x.”
Contour lines not only give you the elevation, but they also relate slope or grade. The more closely spaced the contour lines, the steeper the grade. Going back to Figure 1, compare the “Map View” with the “Profile View.” The profile view is plotted along the red line. Do you see the difference in slope between the more widely spaced contour lines on the left-hand side of the hill’s peak to those on its right? Which route would you take to the top?
Another feature of contour lines is that they always point upstream. Note how the contours in Figure 2 come to a point where they intersect a stream. Lines will also come to a point where there is a drain or washout. Many of these drains will have water flow during wetter periods, even though the maps do not indicate so.
Contour Map Uses
In forestry, topographic maps are essential for proper harvest planning. They are an aid in locating items such as loading areas, new roads, stream crossings, and erosion buffers.
For hikers, topographic maps are a plus when in the backcountry. The elevation data helps determine a trip’s difficulty and the land features displayed are a must for precise navigation.
One of the best types of topographic maps for both professional and recreational uses are the US Geological Survey Topographic series. These are available in both paper and digital formats. You can view these maps and others here: www.gpsvisualizer.com/draw/
Just zoom to the desired location and then go to the top right-hand drop-down menu and select “USGS Topo.”
View the USGS guide to topographic symbols here: http://pubs.usgs.gov/gip/TopographicMapSymbols/topomapsymbols.pdf