11 Topographic maps
Topographic maps add a third dimension to latitude and longitude by showing natural (and cultural) features of the earth’s surface — in particular, elevation. Historically, mapmakers used a variety of methods to indicate elevation. To get a quick sense of elevation, we can use color, as in this map:
This method of showing elevation wouldn’t help much for a closeup map, though — say, for hiking or roadbuilding. For simplicity, clarity, and accuracy, most topographic maps today use contour lines.
Contouring is drawing lines on a two-dimensional grid of numbers that connect points of equal value. In the following grid, for example, I’ve drawn contour lines connecting multiples of 10, so we’d say that the contour interval — the space between the lines is 10:
That’s a simple example. Here’s a more complicated one — can you see the hills and plains?
If you want to use this in your classroom, I’ve provided a blank grid below. Have students try different contour intervals (5 and 10, say) and see how it affects the finished picture. What geographical features seem to appear? Which are more or less prominent depending on the contour interval? (Hat tip to Lorraine Remer for the idea.)
Finding and using topographic maps
On a topographic map, the numbers on the grid correspond to elevation — typically feet or meters above sea level. The contour lines on a topographic map show lines of constant elevation, so it’s easy to see hills, valleys, and plains, and the lines are usually labeled with their elevation.
Topographic maps on the web
The U.S. Geological Survey provides map data along with information on how to get that data in map form. The simplest way is to go to the USGS store and select the Map Locator. Follow the instructions to generate a topographic map of your location (or any other location in the United States). Once you’ve downloaded them, topographic maps can be integrated into any study of history or geography, particularly where terrain is relevant to the events you’re studying. (The Lewis and Clark expedition comes to mind.)
To add a hands-on component to your exploration of topographic mapping, have students make models of a landscape with hills, valleys, and plains. Then measure the elevation of various points on the model, and draw or paint contour lines onto it. (How can you measure the elevation of points on the model? You might design something with two rulers held at a right angle by a protractor, using the vertical ruler to measure height from the desk and the horizontal one to touch the model. Or present it to students as an engineering challenge!)
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