Reading Topo Maps
by Tom Russo
A quick overview of the training might be helpful here. We met for a little more than an hour and had a classroom presentation of basic information about map and compass use, and then we drove over to Bear Canyon trailhead to do a field exercise. The field exercise began with participants dividing up into teams and using resection to find their initial location. Once this was done teams were shown a map of the area on which we had marked certain locations. The teams had to use their knowledge of where they were, their maps and their compasses to navigate to the marked locations and find markers we had placed there.
Surprisingly, the most difficult part of this course appeared to be the resection exercise. Specifically, most of the problem seemed to stem from members having trouble associating the terrain features in front of them with the squiggly lines on the map --- without being certain that a given squiggle is a given hill, it is rather difficult to pinpoint your exact location.
The purpose of this article, then, is to highlight some aspects of topographic map reading to help you think about the problems inherent in using these maps in the field.
Map symbolsI have not yet obtained publisher's permission to reprint the table of map symbols which I used in our training handout, so I'm not including it here. But better than that figure is the US Geological Survey pamphlet "Topographic Map Symbols," which may be obtained for free wherever USGS maps are sold. I highly recommend getting a copy for your map kit.
Topographic Maps, contours, and feature recognitionElevation features are described on maps by use of contour lines. A contour line on a map is the line you would trace out on the terrain if you were to walk along a path of constant elevation. Making the mental translation from contour lines on a map to the terrain around you takes practice, and this article cannot possibly be a substitute for that practice.
Put simply, here's how some commonly observed terrain features translate into contour lines:
- Hills - closed curves of decreasing size. Inner curves are at higher elevation than outer curves, and the peak is within the innermost curve.
- Valleys - these appear as a series of "V" shaped curves. The point of the "V" always points toward the head of the valley, and when there are streams in the valleys the "V" points upstream.
- Saddles - The space between peaks can be recognized by its hourglass shaped contour lines.
- Gentle slopes - slow rises in elevation over a long horizontal distance are represented by widely-spaced contour lines.
- Cliffs - since they represent dramatic changes in elevation over a short horizontal distance, cliffs are represented by closely-spaced contour lines.
Common PitfallsNow that we touched on the easy stuff, it's time to consider the things that we didn't cover in the classroom portion of our training, and which were obviously a problem for team members participating in the training.
One tendency we observed --- and not just in the participants, mind you --- was an eagerness to identify little crosses and elevation markings on the map with prominent pointy things in the field. By doing so, some of our teams managed to "pinpoint" their initial location in a triangle with an area of about a square kilometer, the nearest corner of which was as much as a kilometer away from the true initial location. One might reasonably consider that to be insufficient accuracy. The problem here is in trying to identify a single point on the map to a single point in the world, without considering all the additional shape information that the topo map is providing. In one particular instance, team members were claiming that a particular elevation marking on the map was the highest nearby hill, without noticing that the elevation marking in question was clearly surrounded by contours of higher elevation, meaning that the marked point was really just a knoll on a spur on the side of a much larger hill --- in fact the hill they were trying to identify with that point was a good 200 feet higher in elevation.
Another pitfall caught some of the participants: sometimes a big, prominent hill makes a lousy landmark for a resection exercise. How can that be? Well, if the hill in question is close, relatively flat at the top, and pretty tall then the "peak" you're looking at might not actually be the spot marked with an X on the map. This is because the true peak might be obscured from view, or might be too subtle a "peak" to be detectable (e.g. a 40 foot high knoll in the middle of a 1000 foot circle of relatively flat, tree covered hill). The problem in this case can be avoided by comparing the shape of what you're actually looking at with the contour lines on the map --- seeing that the terrain surrounding what you're calling the "peak" does not have the same shape that the map says it should would be a good indication that you're looking at a different part of the hill than you might have guessed.
A further error we observed was in attempting to identify really neat curvy bits of contour lines with terrain features --- the base of a spur, or the mouth of a draw would be good examples. The problem here is that it's rather difficult to pinpoint such an indistinct terrain feature: they tend to be at points where the mountain is flattening out, and so are more "area features" than "point features" and what looks like a nice sharp "V" on the map may not be so distinct at the base of the hill. Remember that it's hard to see contour lines in the real world --- perhaps others have better eyesight, but as far as I can tell God drew them in in disappearing ink.
When reading a topo map, try not to look at the terrain around you and make quick identifications of peaks. Rather, look at the map and try to reconstruct the vertical shape of the terrain and match that to the things around you.
In a future minilesson, possibly in an alternate universe where there are 27 hours in a day, I would like to print up photographs of some of our training areas and corresponding sections of USGS quadrangles. But in the meantime, try to get in the habit of bringing a good topo map along with you when you stroll around familiar areas of the Sandias. Practice reading the shape of terrain from the contour lines in addition to trying to pick out familiar peaks. It's harder than it sounds, and sadly a written, sparsely illustrated lesson cannot do the subject justice.
Self Test on Topo Map ReadingNow that you've seen how contours relate to terrain features, try this self-test. Match the contours on the left with the terrain on the right.
Image source: "Be Expert with Map and Compass" by Bjorn Kjellstrom
Copyright and Disclaimer:: The contents of this website and the newsletter contained here are copyright © 1996-2003 by their respective authors or by Cibola Search and Rescue, Inc., and individual articles represent the opinions of the author. Cibola SAR makes no representation, express or implied, with regard to the accuracy of the information contained in these articles, and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Articles made available at this website may be reproduced, with attribution given to Cibola SAR and the author, by any member of the Search and Rescue community for use in other team's publications.