How to Use a Map

Page 4

 

Contour

 

As was stated earlier, the earth is not smooth like a cue ball. The land is covered with mountains, valleys, and other types of terrain. We also said that contour lines show, not only the height of these features, but also the shape. Map makers have come up with a standard set of "terrain features" used on maps to depict the shape of the terrain.

 

 Be advised, however, that no two terrain features are exactly the same, just as no two mountains are the same. But, being able to recognize the standard terrain features will allow you (with practice) to identify whether a terrain feature on the map is a hill or a valley or one of the other terrain features, but also allow you to identify that same terrain feature on the ground using a technique called "terrain association." Terrain association also gives you the ability to look at a terrain feature on the ground, and identify it on the map. More on this later.

 

Before we get into the specifics about terrain features, let's talk a little more about contour lines. "A contour line is a line representing an imaginary line on the ground along which all points are at the same elevation."  This means that when you know the elevation of a spot on a contour line, then you will be at the same elevation anywhere along that same contour line.

 

So how is the elevation of a contour line determined?

 

There are three types of contour lines. "Contour Lines", "Index Contour Lines", and "Supplemental Contour Lines." If you look close at a map, you will see that some contour lines are drawn "heavier." These heavier contour lines are the Index Contour Lines.

 

Contour Lines A2.jpg (183852 bytes)

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Look at an Index Contour Line. With a sharp pencil, follow that Index Contour Line (either direction) and eventually you will see a number written on that line. This is the elevation of that Index Contour Line. If you look at the contour interval at the bottom center of the margin, you will see the unit of measure. It will say either feet or meters. Let's say that the number you found on the Index Contour was 2200 and the contour interval is 40 feet. Then the elevation of that Index Contour Line is 2,200 feet.

 

You will notice that Index Contour Lines are drawn every fifth line. The lines drawn between the Index Contour Lines are standard "contour lines." The distance between each contour line is the contour interval, which in our example is 40 feet. Since the contour interval is 40 feet, and there are five "intervals" between Index Contour Lines, then that means there is 200 feet of elevation change between two Index Contour Lines, and 40 feet of elevation change between two adjacent contour lines. 

 

Contour Lines A3.jpg (137939 bytes)

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Each contour line's elevation can be determined if you know:

 

  1. The elevation of the nearest Index Contour Line.

  2. Whether the contour line in question is uphill or downhill from the Index Contour Line.

 

We will be getting into determining the direction of elevation in a few minutes. For now, let's look at an example of a contour line that is uphill two intervals from an Index Contour Line who's elevation is 2400 feet and has a contour interval of 40 feet. The total elevation change would be 80 feet (2 x 40 feet). Since it is uphill from the Index Contour, then the elevation change is added to the Index Contour Line's elevation, making the elevation of the contour line in question exactly 2480 feet (2400 + 80).

 

Contour Lines A4.jpg (129176 bytes)

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Let's say that you are trying to determine the exact elevation of a spot on a map, and that spot is located directly between two contour lines. What would the elevation be at that point? If you still heading uphill from the Index Contour Line, then add 1/2 the contour interval (in our example, that would be 20 feet) to the elevation of the nearest downhill contour line. 

 

Contour Lines A5.jpg (143018 bytes)

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If the contour line you are looking at is downhill from your selected Index Contour, then you would subtract the contour interval instead of adding it. 

 

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