Why are maps drawn from above (vertically) more accurate for measuring distance than maps drawn from the side (oblique)?

Techniques for depicting terrain from directly above were developed for two primary reasons. First, the oblique view inherently hides some map features; a vertical view, by contrast, offers a view of all landscape features within the map frame. The vertical view also allows the map maker to position features appropriately in geographic space—thus providing concrete spatial information, rather than a more artistic visual representation.

In the vertical view, terrain is typically represented with contour lines. Contour lines drawn on a map connect points of equivalent elevation. Figure 6.4.1 demonstrates how contour lines relate to the landscape from which they are derived—note that the bottom image is a 2D rendering of what is presumed to be a regularly-shaped mountain feature.

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