# 3.6: Drawing Contour Lines and Topographic Profiles

## Overview

Constructing a topographic map by drawing in contours can be easily done if a person remembers the following rules regarding contour lines:

1. Contour lines represent lines connecting points of equal elevation above sea level
2. Contour lines never cross, split, or die off
3. Contour intervals must be obeyed, therefore the contour line elevations can only be multiples of the contour interval
4. Contour lines make a “v” pattern as they cross streams and rivers, and the “v” always points towards the upstream direction.

As you draw a contour line on a map you will notice that the elevations on one side of your line will be lower elevations, and elevations on the other side of your line will be higher elevations. Once your contour lines are drawn, you will notice that you had to draw some lines closer together in some areas and wider apart in other areas and that you may have even enclosed an area by drawing a contour line in a circular pattern. These circular patterns indicate hilltops, like in the diagram below (Figure 3.6). To illustrate what these hills look like in profile (or, how they would look if you saw them while standing on the ground and looking at them from a distance), you can draw what is known as a topographic profile. Essentially a topographic profile is a side image of a topographic map, but the image is only a representation of the area shown on the line on the topographic map (line A-B on Figure 3.6). To construct a profile, you need graph paper, a ruler, and a pencil. You want to have the y-axis of the graph paper represent the elevations of the contour lines that intersect your drawn line (line A-B in this case). By using a ruler, you can transfer these elevation points from your topographic map straight down onto your graph paper such as shown in Figure 3.6. Be sure to only plot those elevations that are at the intersection of the contour line with line A-B. Once your points are plotted on the graph paper, you simply connect the dots. As a rule, hilltops will be slightly rounded to show a slight increase in elevation to represent the crest of the hill, but be careful not to draw the hilltop too high on your graph paper. For example, the first hill on the left has a top contour line of 50ft. Because there isn’t a 60ft contour line on this hilltop, we know that the hill’s highest point (the crest) is some elevation between 50 and 60ft.

When connecting the points on your graph paper in the area between the two hills in Figure 3.6, you again want to round out the area to represent the base of your valley between the hills, but be careful not to make the valley floor too deep, as according to the topographic map the elevation is below 40ft, but not as low as 30ft.

If you examine the graph showing the topographic profile in Figure 3.6, can you imagine what would happen to your profile if we changed the spacing for elevations on the y-axis? When the vertical dimension of your graph is different from the horizontal dimension on your map, you may end up with a graph that shows a vertical exaggeration and the features of the Earth represented by your topographic profile may be deformed such as in Figure 3.7. Sometimes vertical exaggeration is desired but, in some cases, you may not want it. To avoid having your profile distorted so that it accurately conveys what the surface of the Earth really looks like in profile, you will want both the vertical and horizontal scales to match. For example, if your map scale is 1 inch = 50 ft, then one inch on your graph’s y-axis should only represent 50ft of elevation. If the topographic map in Figure 3.6 has a fractional scale of 1:12,000 then 1 inch is equal to 12,000 inches or 1000ft; this 1inch = 1000ft equivalency is for the horizontal scale. When we hold a ruler to the y-axis of the topographic profile in Figure 3.6, we see that 0.5 inches = 50ft, which means 1 inch = 100ft on the vertical scale. To calculate the vertical exaggeration in the topographic profile shown in Figure 3.6 we divide the horizontal scale by the vertical scale: ($$\frac{\frac{1000\;ft}{1\;inch})}{(\frac{100\;ft}{inch}}) = 10$$. Therefore the topographic profile in Figure 3.6 represents a profile of the map surface (along the A-B line) that has been vertically exaggerated by 10 times (10X).

## Answers to Practice Lab Exercises, Parts A and B

1. 76°15’00” W longitude
2. 43°00’00” N latitude
3. 2000 ft
4. 1 ft = 24,000 ft, and 24,000 ft x ($$\frac{1\;km}{3280\;ft}$$) = 7.32 km
5. South