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How topographic maps are constructed. A contour is
a line drawn where any plane (like B or C) intersects the land surface.
Where the plane of the water (A) intersects the island, it forms the 0-foot
contour line. Here, elevation = 0, because it is sea level. Where the 50-foot-higher
plane B intersects the land surface, it forms the 50-foot contour line.
Where the 100-foot-high plane C intersects the land surface, it forms the
100-foot contour line. D is the resulting topographic map of the island.
It was made using the contours in 50-foot increments above mean sea level. |