Constructing a topographic profile

Topographic Profiles

The purpose of a topographic profile is to provide a representative appreciation in the vertical dimension of the topography within a map area. Thus the topographic profile provides a "side-looking" view of the topography and assists in visualizing the relief of the map area.

Use the following procedure in constructing topographic profiles. Refer to Figure 7-7 as you read through the procedure.

1 . Select a line for the profile (line AA' in Fig. 7-7A).

2. Place the straight-edge of a strip of paper along and above this line (Fig. 7-7B).

3. Mark on the paper the elevation at the exact point that contour lines and areas of high or low relief (hills and valleys) cross the selected profile line. Knowledge of the elevations of hills and valleys that lie between contour lines will aid in reconstructing the topography between contour lines (Fig. 7-7B).

4. Determine the scale of the vertical exaggeration (see below) to be used and place a grid which represents this scale on a separate piece of paper. Mark the divisions of vertical scale in the units of the contour lines.

5. Place the strip of paper containing the information from the profile line above the paper containing the grid. Project the position of each contour line and each marked topographic feature downward to Its proper elevation (Fig. 7-7C).

6. Connect all of the successive points on the grid by a smooth, curved, and unbroken line. The result is a topographic profile along the line selected for the profile (Fig. 7-7C).

Vertical exaggeration (VE)

The horizontal scale of a topographic profile is the same as the map from which ft is constructed. However, because of the scale of most maps, the small amount of variation in elevation at map scale will mask most details of topography. For this reason, topographic profiles are usually constructed with some amount of vertical exaggeration (VE) to emphasize the details of topography. The vertical scale of the topographic profile is smaller than that of the map. The amount of VE must be noted on the topographic profile. The VE is determined by the following steps.

1 . Determine the number of feet represented by one inch on the vertical axis of the topographic profile. In Figure 7-7C one inch represents 100 ft or 1:1200.

2. The horizontal scale of the profile is the same as the map, 1:24,000.

3. The VE is calculated by dividing the horizontal scale by the vertical scale.

VE = horizontal (map) scale/vertical scale

VE = 24,000/1200

VE = 20x

The VE of 20 indicates that the topography illustrated in Figure 7-7C is exaggerated by 20 times that of the original map.