Engineering geology
Topographic profile_google

TOPOGRAPHIC PROFILES AND VERTICAL EXAGGERATION

A topographic map provides an overhead (aerial) view of an area, depicting features and relief by means of its symbols and contour lines. Occasionally a cross section of the topography is useful. Atopographic profile is a cross section that shows the elevations and slopes along a given line (Figure 1). Follow these steps and Figure 1 to construct a topographic profile:

Step 1: On the map, draw a line of section along which the profile is to be constructed. Label the section line A–A’. Be sure that the line intersects all of the features (ridges, valleys, streams, etc.) that you wish the profile to show.

Step 2: On a strip of paper placed along section line A–A’, make tick marks at each place where a contour line intersects the section line and note the elevation at the tick marks. Also note the location and elevation of points A, A’, and any streams crossed.

Step 3: Draw the profile. On a separate sheet of paper, draw a series of equally spaced parallel lines that are the same length as the line of section (graph paper can be used). Each horizontal line on this sheet represents a constant elevation and therefore corresponds to a contour line. The total number of horizontal lines that you need, and their elevations, depends on the total relief along the line of section and on whether you make the space between the lines equal to the contour interval, or to multiples of it (vertical exaggeration, which will be discussed shortly). Label your lines so that the highest and lowest elevations along the line of section will be within the grid.
Then, take the strip of paper you marked in Step 2 and place it along the base of your profile. Mark a dot on the grid above it for each elevation. Smoothly connect these dots to complete the topographic profile. (This line should not make angular bends. Make it a smoothly curving line that reflects the relief of the land surface along the line of section.)

Step 4: The vertical scale of your profile will vary greatly depending on how you draw the grid. It almost certainly will be larger than the horizontal scale of the map. This difference causes an exaggeration in the vertical dimension. Such exaggeration almost always is necessary to construct a readable profile, for without vertical exaggeration, the profile might be so shallow that only the highest peaks would be visible.
Calculate the vertical exaggeration by one of two methods. You can divide the horizontal ratio scale (1:24,000) by the vertical ratio scale (1:1440), which reduces to 24,000/1440, which reduces to 16.7 (Method 1, Step 4, Figure 1). Or you can divide the vertical fractional scale (1/1440) by the horizontal fractional scale (1/24,000), which reduces to 24,000/1440, which reduces to 16.7 (Method 2, Step 4, Figure 1). The number 16.7 (usually written 16.7 3) indicates that the relief shown on the profile is 16.7 times greater than the true relief. This makes the slopes on the profile 16.7 times steeper than the corresponding real slopes on the ground.

FIGURE 1 – Topographic profile construction and vertical exaggeration. Shown are a topographic map (Step 1), topographic profile constructed along line A–A’ (Steps 2 and 3), and calculation of vertical exaggeration (Step 4). Step 1—Select two points (A, A’), and the line between them (line A–A’), along which you want to construct a topographic profile. Step 2 To construct the profile, the edge of a strip of paper was placed along line A–A’ on the topographic map. A tick mark was then placed on the edge of the paper at each point where a contour line and stream intersected the edge of the paper. The elevation represented by each contour line was noted on its corresponding tick mark. Step 3—The edge of the strip of paper (with tick marks and elevations) was placed along the bottom line of a piece of lined paper, and the lined paper was graduated for elevations (along its right margin). A black dot was placed on the profile above each tick mark at the elevation noted on the tick mark. The black dots were then connected with a smooth line to complete the topographic profile. Step 4—Vertical exaggeration of the profile was calculated using either of two methods. Thus, the vertical dimension of this profile is exaggerated (stretched) to 16.7 times greater than it actually appears in nature compared to the horizontal/map dimension.

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