Contour lines

INTRODUCTION TO MAPS AND CONTOUR LINES

There are a number of ways to navigate in the wilderness. Of course, you can find the North Star and use that, with a high degree of imprecision, to get your bearings.

Some folklore that I followed, when I was a kid, was that moss grows on the north side of trees. It always seemed to be true when I checked it out. My perception of North always matched the side of the tree on which the moss grew the thickest. Oops! Now that I think of it, my perception of North was probably strongly influenced by my folk knowledge of where the moss was supposed to grow. I wasn’t much into carrying a compass back then.

Of course, now the thing to do is take a GPS along with you. And, as long as you have charged batteries and you haven’t dropped your device into a brook along the way, this is a great way to navigate with ease.

CONTOUR LINES

But, let’s take a look at one feature of that old standard of navigation, the topographical map. What I want to talk about is those curvy lines that you see on the map, officially known as contour lines, also referred to in this post as “contours”.

Topographical maps purport to represent not only two dimensions, but three. Not only do they depict North, South, East, and West and all points between, but also relative measurements starting from the center of the earth and projecting out. In short, a topo map claims to represent also mountains hills and valleys. In other words, it tries to represent elevations.

But, a map is two-dimensional. So, how do you represent elevations on a two-dimensional map? The answer is “contour lines” (contours).

A DEFINITION

A definition of a contour line is a line that connects points of equal elevation. The word “contour” gives us a hint that these lines are supposed to represent the contours of the landscape – the valleys, the hills, the cliffs and the peaks.

LEARNING TO READ CONTOUR LINES

So, how do you read the contour lines? Let me see what kind of a job I can do teaching you with the map that I’ve included below.

Contour lines on a topographical map

First, we’ll start with what I call “index contours”. One of these is just below the center of the map (above). It is a thicker line compared with other lines on the map. It also has an elevation printed in the contour. In this case you can see that the elevation for this index contour is 2500, which represents 2500 feet above sea level.

So, can you find another index contour? Well, there are several on this map. One is just to the left of the intersection of the North-South and the East-West lines (blue).

The vertical distance between these contours is fixed by the cartographer and is typically 500 feet. We’ll assume that’s the case for this map so that we can discuss further how contour lines work. So, if you look at the map and locate the first index contour (the one including “2500″) and go to the next contour line on any side, you know that all points on that line are at 2000 feet in elevation (above sea level).

So, taking this information we can get a rough indication of the elevation at any point on the map. Let’s take for instance the town of Kerriston near the bottom right corner of the map. At what elevation would this town be? Well, the contour lines will help us answer this question. Starting at our index contour we go east to the next line which represents an elevation of 2000 feet and one more line to the east which represents an elevation of 1500 feet. Kerriston, according to this rough calculation, is somewhere below 1500 feet and above 1000 feet in elevation. Checking my work, I found online that the town of Kerriston, a ghost town, is at an elevation of 1266 feet. Our calculation worked quite well.

One more thing that contour lines can tell us is the relative steepness of the terrain. Take a look at the hill on which we find the index contour that we have been working with (just below the midpoint of the map). Notice that the steepest part of this hill is on the northeast side. We can tell by the fact that the lines are closer together than on the opposite side (towards the town of Hobart). When you see contour lines practically on top of each other on a topographical map, you’ve got a representation of a cliff.

The contour lines on a topographical map are designed to help people navigate safely in the wilderness. Knowing how to read them could save your life someday. But, reading contours on a map and comparing them to the surrounding terrain can also be just plain fun.

By Richard Davidian, PhD

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