The Drainage Map Tab.
The drainage map is used in conjunction with the rainfall map to
determine the biomes present in the tile.

The Zoom Level control will zoom in all maps associated with this tab.
Zooming in will change the size of the edit brush so that the edit
reticle cursor always shows the extent of the brushes effect.
The ‘Percent Below Dune/Swamp Threshold’ slider
controls the threshold of the lowest category of drainage. Sand Desert
and Swamp will occur here depending on the rainfall amount.
The ‘Percent Below Rocky/Grass Threshold’ slider
controls the next threshold, generally rock desert and grassland appear
below this drainage level.
The ‘Percent Below Hills Threshold’ slider controls
the third drainage threshold, below this level is usually hills and
above this level can be the sharp spiky badlands type or forest.
As a starting point for the drainage map, we calculate the standard
deviation over a small area to judge its flatness or steepness.
The ‘Filter Size’ control determines the size of
the area used to determine the flatness of a tile. Larger values
generate smoother results but take longer to calculate.
The ‘Elevation Standard Deviation Map Weight’
slider controls the amount of the standard deviation map that is added
to the final drainage map.
The noise map control is the same as on other tabs.
The ‘weight’ slider governs how much of this noise
map is added to the final map.
The ‘Feature Size’ slider adjusts the size of the
features in the noise field. The higher the number, the smaller the
features get. If that seems awkward, the slider is actually controlling
‘frequency’ directly, and size indirectly as a
result of frequency.
The ‘Smoothness’ slider adjusts the smoothness of
the noise field.
The ‘Generate New’ buttons scramble the noise
inputs to create a new noise field.
You may also click and drag on the noise fields to adjust their
position in case you want a certain feature to appear in a specific
location.