What happens when the points of equal value for a contour line do not form a line but rather an area?

Question

The whole idea of a contour map assumes that the value of what is being measured by the contours is constantly changing along the lines perpendicular to the contour line. But this doesn't have to be true.

For example, what would happen on a contour map that has contour lines for elevations 2 ft, 4 ft, 8 ft, and 10 ft if there was a round plateau 5 square miles in size so that every point in this 5 square mile area was at exactly 8 ft? A contour line would not suffice in depicting all the areas on the map that are exactly 8 ft high.

Answer 1

The contour line would run at the point where 7.99 became 8.0 feet.



Inside that area would be spot heights.



All of them in that area would be 8.0



Normally a spot height shows the actual highest elevation.



So a hill passing the 2000 feet contour but not reaching the 2100 line would have a spot height of 2076 indicating the summit.

Answer 2

In your example the 5 square mile area is an absolutely flat plateau (impossible practically). Where the value of 8 ft changes is the Contour. I don't think you would say that the whole Earth is at 8 ft high +MSL (in that case the whole coast of land is the contour that is otherwise a Contour 0 ft high).

Answer 3

That is correct. A contour line is really a boundary between points of a higher elevation separated from points of a lower elevation. When you see a countour line encircling an area, it is possible that the elevation of that area is very close to the contour's elevation. It could be statistically flat. You still have just a contour line delineating the boundaries. You have to assume that the area could be flat.