Average altitude (DM parameter)

The 'average altitude' slider in the design map parameters wizard determines the average height of the terrain, and consequently the fraction of land above sea-level.

This is demonstrated below, where where I've made terrain with average altitude settings from 0% up to 100%:

Average altitude	Design map	Heightfield/Water map	3D render
0% (left, sea)
25%
50% (center)
75% (default)
100% (right, land)