Topographic maps are not just used for determining
elevation, they can also be used to help visualize topography. The
key is to study the pattern of the contour lines, not just the elevation
they represent. One of the most basic topographic observation that
can be made is the gradient (or slope) of the ground surface. High
(or steep) gradients occur in areas where there is a large change in
elevation over a short distance. Low (or gentle) gradients occur
where there is little change in elevation over he same distance.
Gradients are obviously relative. What would be considered steep
in some areas (like Ohio) might be considered gentle in another (like
Montana). however we can still compare gradients between different
parts of a map.
On a topographic map the amount of elevation change is
related to the number of contour lines. Using the same contour
interval the more contour lines over the same distance indicates a
steeper slope. As a result areas of a map where the contour lines
are close together indicate steeper slopes. Areas with widely
spaced contour lines are gentle slopes. The map below examples of
areas with steep and gentle gradient. Note the difference in
contour line spacing between the two areas.