Ever look at a trail map and wonder what all of the squiggly brown lines are for or what they mean? These lines are referred to as contour lines and they represent elevation changes on the map. When you look closely you can start to decipher where these lines represent the terrain like a hill, mountain, canyon, cliff or the plains. Each of these lines follows an exact elevation measure in either feet or meters above a constant, sea level.
Check out the example map to the left. Upon closer examination you will notice that some of the contour lines are double the thickness of the other lines. These double-thick lines are referred to as the index lines and the thinner lines are called intermediate lines. When you follow along an index line you will find a printed elevation. In the case of the example map we have two index lines that are both labeled as 1000. This tells us that these lines are 1000 feet above sea level. If we had a full map we could find the bar scale, which is usually located in the margin, and printed near it you we would find the contour interval. This would tell us the change in elevation between each of the contour lines. Since we don’t have the full map we can use other features on the map to figure out the contour interval. Near the left center of the map we have a peak. The top of the peak has a small triangle and to the left of that it is marked 1505. This indicates that the top of the peak is 1505 feet above sea level. Since there are 5 contour lines from the peak to the index line that is marked 1000, we can determine that the contour interval is 100 feet. Now you know that as you cross over from one contour line to the next you are either gaining or losing 100 feet of elevation each time.
In the bullets below I’ve included a few more tip about reading map contour lines. If you are really interested in mastering map reading, start with the book Be Expert With Map & Compass, by Bjorn Kjellstrom, it’s a great learning tool. Not only does it explain maps, map reading and compasses in plain English, but it also includes exercises and tests to help reinforce the lessons.
Contour Line Tips:
- The closer together the contour lines the steeper the slope
- There is no beginning no end to a contour line, they are a continuous loop, if you were able to walk along a contour line you would neither gain nor lose elevation and would eventually return to the place that you started
- Mountain peaks are usually represented by a triangle or an X and show a printed elevation next to them
- Depressions are usually indicated by making small hash marks on the downhill side of the depression, pointing toward center (check out the lower right corner of the example map)
- If your path is regularly crossing contour lines of increasing elevation you are going uphill and conversely if the elevation is decreasing you are going downhill
- Generally, when contour lines cross a stream, river, ravine or valley they take on the shape of a V with the point of the V pointing uphill and when they indicate a spur or the ridge on a hill the take on the shape of a U, with the curved end of the U pointing downhill
- Another way to determine the contour interval is to find two adjacent index lines, subtract the smaller elevation from the larger and then divide this by the number of contour gaps you cross going from one index to the next
Have fun exploring the unknown!