LAB EXERCISE 9.2 Determining Elevations from Topographic Maps

In Figure 9.7, what is the elevation of the lowest point in the crater? Explain how you made this decision.

225 feet; half the contour interval lower than 250 feet.

A red "X" in Figure 9.5 marks a spot on a contour line along the shore of Baskahegan Lake, but the printed DEM doesn't say anything about its elevation. Use the topographic map (Fig. 9.4) to estimate the elevation of this point as accurately as possible. Write your answer in feet.

420

in Figure 9.4, what is the elevation of the intersection of U.S. Route 1 and Maine Route 6 in Topsfield? Write your answer in feet.

490

What is the relief (difference in elevation) between Little Tomah Lake and the top of Tomah Mountain (look for "Tomah Mtn")? Write your answer in feet.

540

In Figure 9.4, what is the elevation of Malcome Bog? Write your answer in feet.

550

In Figure 9.4, what is the elevation of the highest point on Hunt Ridge (keep in mind step #4 from the first question)?

730

What is the relief between the surface of East Musquash Lake and the intersection between U.S. Route 1 and Maine Route 6 in Topsfield? Write your answer in feet.

120

In Figure 9.4, what is the elevation of the highest point on Farrow Mountain? Write your answer in feet.

990

To determine the elevation of a point on a topographic map: 1. Determine the contour interval for the map. 2. Find the known elevation closest to the point. This may be a benchmark or a contour line. Remember that every fifth contour line is a heavier index contour, but you may have to follow contour lines some distance before finding an elevation label. 3. Determine whether the point is higher or lower than the known elevation by examining the sequence of adjacent contour line elevations and by taking note of nearby feature names, such as "fire tower," "valley," and so forth. 4. Interpolate between the contour lines that bracket the point to get the elevation. For consistency, assume that elevations between contour lines are 1/2 a contour interval higher than elevation of the lower line. For example, if a location point is between the 150 feet and 200 feet contour lines than the elevation is 175 feet (1/2 of 50 feet is 25 feet; 25 feet added to 150=175 feet). Do the same when trying to determine the elevation of a hill top. In the case of a depression, you instead subtract, because you're going down in elevation. Figure 9.7 Hachured contour lines indicate depressions.

The highest point of the rim of the crater is about 350