Why are the contour intervals on a topographic map smaller on a plain than a mountainous area?

Answer 1

Nice. Now, to answer your question...

The previous post pointed out that there will be more lines in a mountainous area than on a plain, IF the contour intervals are the same.

Since the lines are further apart on a plain, the mapmakers have the opportunity to decrease the contour interval to provide more detail. In a mountainous area, decreasing the contour interval would make the map unreadable, since there would be too many lines.

Answer 2

If you look at a topographic map, the solid, curving lines are contour lines that indicate the elevation and shape of terrain. The elevation change between any two adjacent countour lines (the contour interval) is the same everywhere on a given map. If the contour interval is 10 feet, the elevation changes 10 feet within these adjacent lines. Contour lines close together indicate steep mountainsides or cliffs. Gentle sloping hills or flat plains have contours farther apart from each other. Experienced map readers can look at the contours and visualize the three-dimensional shape of the land. “Cutting” a line through the contours enables the map reader to draw a profile of that section of the mountain or plateau. Hachure marks along a contour show depressions, for example, of a crater or the mouth of a volcano.

Answer 3

Specifically, on a plain where relief is small, the contour interval will be 10 feet or so, so the relief of the land can be seen. With a 20 foot contour interval on flat land, the terrain, drainage patterns, etc., will be lost. As pointed out, on a steep slope, the elevation change will put 10 contour lines too close together to be readable. Changing the contour interval to 20 feet removes half the contours. Some terrains are so rugged, 50 foot contours have to be used as 20 foot lines are too close together. On very steep slopes, cartographers will remove the interim contours and only show the index contours.

Contour interval is based on the relative relief to be shown.