Land gradient support: upstream and downstream for rivers

[oubiwann]

Currently, we can tell a player that in a given 25 square kilometers of world space there is a river; by looking at surrounding pixels, we should be able to say if there are other rivers/tributaries that join it/split from it "near by"; but there's currently no way of telling a player that:

"To the North, the river is upstream, and to the West, the river is downstream; that there is a tributary to the East, coming from the hills, and that a gorge to the south causes the river to split into two (the second downstream branch being to the south)."

• How can we support the inclusion of land gradients, so that the above example would be possible?
• We currently have a global river overlay: can we use land gradient data to accurately and easily determine overall downstream for a river? (A local "reversal" of direction could be an indication of severe river bending.)
• We currently have elevation bitmaps, but general gradient behaviour is very tricky to calculate (and, in fact, we used an external application to generate rivers, lakes, water basins, etc.); is there a good way to do this? If so, does this functionality need to be moved into the image server project? (See Investigate moving image analysis/generation into separate service #30)