How to Draw a Cross-Section from a Topographic Map
I’ve wanted to write this blog post for a long time, but this weekend pushed it forward on my list because of something my daughter said to me. As some of you who follow me on social media may know, my two kids are runners. This past weekend my daughter competed in the Region 4 USATF Junior Olympic Cross Country meet in Tallahassee, Florida. She had a great meet and qualified to run at the National JO meet in December.
As I pulled up the course maps for December in Alabama, I noticed that they were unlike any course map I’ve seen over the years. There are topographic lines on the map! For once, you can look at the map and see where the hills and valleys are on the cross country course. The first thing I thought to do was to make a cross-section of my daughter’s course map to see if it was flat or hilly. We live in coastal South Carolina so running hills can be tough on our athletes. When I mentioned making the cross-section, my daughter said, “you are the only person who would think to make a cross-section of a cross country course.” Really? Certainly, there is at least one other geologist parent attending that meet who whipped up a quick cross-section. Much to our delight, her 3K course looks relatively flat. Yippie!
Making a cross-section from a topographic is not too difficult and can come in handy for hiking, driving, biking, walking and yes, running. All you need is a topographic map of the area, a piece of graph paper, a ruler, and a pencil. There are some great online resources for obtaining topographic maps. I will link to one I like at the bottom of the post.
For this demonstration, I am using Stone Mountain, Georgia because it has a significant elevation difference than the cross country course. Here is a topographic map of Stone Mountain.
There are three basic steps to making the cross-section once you have your map.
Step 1:
The first step to complete to create your cross-section is to choose a line across the map where you want to view the elevation changes. This could be the area you plan to hike or as in this example here, a line through the highest part of the mountain so that we can get a really good visual of the mountain’s elevation.
Once you choose the area for your cross-section, draw that line on the topographic map using a ruler. Geologists will label this cross-section line using letter designations such as A – A’. You could use “A – B” or “Start – Finish” or anything else you choose.
Step 2:
The second step is to take a piece of graph paper and fold it along one of the horizontal grid lines. I like to choose an area toward the bottom of the graph paper to give myself plenty of room to work. Lay the edge of the graph paper along your cross-section line then mark the locations of A and A’ on your paper. Mark each topographic line or each index line (the darker topographic lines) if there are more topo lines that you can easily use in the construction of your cross-section, on the paper and label the mark with the elevation of that line. In this example, each index line is at a 100-foot interval while the lines in between are at intervals of 20 feet. Because Stone Mountain is very steep, I can mark only the index lines and still have a nice representation of the height of the mountain. If your area is flatter, you may want to use all of the topographic lines to accurately see the elevation.
Step 3:
Once you mark each topographic line and its corresponding elevation on your graph paper (as shown), unfold the graph paper and make a lined graph above your marks. Mark the elevation of each topographic line on the graph directly above the mark for the line. Once you have all of the dots placed on the graph, simply connect the dots to see a representation of the elevation of the area as shown in the image.
I am not the best artist in the world so your drawing may be a lot prettier than mine, but this will give you an idea of how you can use a topographic map to quickly drawing a cross-section of the elevation of an area. Give this a try the next time you are going for a walk, hike or run in a new area to give yourself an idea of the elevation of the terrain the area.
Topographic Map Resource:
https://www.mytopo.com/maps/
Filed under: Geology / Earth Science, Geology Education on December 6th, 2016
thank you .. it is helpful