On Earth, the elevations can take on either positive or negative (below sea level) values, and are bimodal due to the contrast between the continents and oceans. On other planets within this solar system, elevations are typically unimodal, owing to the lack of oceans on those bodies.
A hypsometric curve is a histogram or cumulative distribution function of elevations in a geographical area. Differences in hypsometric curves between landscapes arise because the geomorphic processes that shape the landscape may be different.
When drawn as a 2-dimensional histogram, a hypsometric curve displays the elevation (y) on the vertical, y-axis and area above the corresponding elevation (x) on the horizontal or x-axis. The curve can also be shown in non-dimensional or standardized form by scaling elevation and area by the maximum values. The non-dimensional hypsometric curve provides a hydrologist or a geomorphologist with a way to assess the similarity of watersheds -- and is one of several characteristics used for doing so. The hypsometric integral is a summary measure of the shape of the hypsometric curve.
where a, d and z are fitting parameters. Subsequent research using two-dimensional landscape evolution models has called the general applicability of this fit into question, as well as the capability of the hypsometric curve to deal with scale-dependent effects.