Geoscientist’s Toolkit: Topographic Maps

Topographic map, excerpt from the USGS 7.5' Series (Winona West Quadrangle)
Topographic map, excerpt from the USGS 7.5′ Series (Winona West Quadrangle (31 MB PDF))

Topographic maps (sometimes just “topo maps”) can tell a lot about a place. They record the varying heights of land, from which inferences can be made about those places and their geology. For instance, despite the claims many people make about Minnesota being a flat place, the map above shows something quite different. Sure, there are no 4 km tall peaks here, but this map contains a 500′ embankment, and many stream channels cutting 300′ down from the hilltops.

With the right perspective, understanding a topographic map is fairly straightforward. Let me tell you a story about topo maps.

When I was in eighth grade, my science teacher, Ms. Fuller, was teaching us about maps and mapping. Rather than just looking at maps, we were going to make maps. On a cool, cloudy fall day, we all loaded onto a school bus for a field trip to a nearby park with a lake in it. The students had been divided into teams of three, and each was given a pair of metersticks and a long tape measure.

As we walked along the path at the water’s edge, every so often (50-100′) a team would be assigned to measure a profile up the hill. One person, Alice, would hold a meterstick upright at the path, and another, Bob, would walk directly away from the lake until his feet were level with the top of Alice’s meterstick. Here, Bob would hold his meterstick upright. The third team member, Eve, then measured and recorded the distance between the two metersticks, from the top of Alice’s to the base of Bob’s. Once that had been done, Alice would make her way through the brush, past Bob, and up the hill until her feet were level with the top of his meterstick. The distance between them was measured, and the process repeated until they had gone ~75 m away from the lake and reached the top of the hill and the edge of the park. If they had extra time, they measured another profile from a different location around the lake.

Back in the classroom, Ms. Fuller compiled the profile data into a topographic map, and added in bathymetric (depth beneath the lake surface) data from an outside source (pardon the pun). You may think that just took the interesting part out of the exercise, but let me assure you, it did not!

The following day in class, we were given a large (18″x18″?) printed copy of the topographic map. On it was a North arrow, and a bold line indicating the edge of the lake. Now the challenge was to build a model of the lake and its surroundings.

We spent the day cutting out pieces of cardboard to match the contour lines. Or rather, we cut the insides out of 18″x18″ cardboard pieces, since the lake area was generally bowl-shaped. Each layer represented a certain amount of height, probably around 3 m. In another class period or two, we had our cardboard pieces fully cut out and glued together, giving us a 3D model of the area. All that was left was to make it look nice. The water was painted one color (typically blue or white, depending on whether we were feeling wintry), and the land a different color (usually green). North arrows were added (for full points), as were horizontal and vertical scale bars (the two scales being different).

Contours on topographic maps represent places where the surface of the Earth passes through a geometric plane for a certain elevation. It’s a slice. Give the slice a width, such as the thickness of corrugated cardboard, and by putting contours together you can arrive at a model of the area.

Places where contour lines are close together are very steep: there is a great change in elevation in a short distance. Areas with few contour lines and large spacing between them are quite flat.

If you have a moment, find your local topographic map, and see what it looks like (US topo maps can be downloaded from the USGS). Alternately, you can enable the terrain overlay on Google Maps (in map mode, not satellite), but you don’t get the excitement of the quadrangle names, the 1:24,000 scale, and the magnetic north arrow.

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