# ArcGIS - Flow Direction (Algorithm) [closed]

I am curious about the algorithm of Flow Direction in ArcGIS. Recently I am studying about Hidrology, and I have some questions regard to Flow Direction in GIS.

The size of this cell is 100m x 100m, with the radius of 10km for the raster data model. I have used the Flow Direction. The result showed that the value of one circle is 1. I am curious about this result.

1. Since the value of the circle is same, how the Flow Direction result could came out like that?
2. what kind of logarithm is that? I have tried to find the answer in here: (http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z00000052000000.htm). But I still cannot understand. Would you explain it to me?

3. Using DEM, finally i got this value. But my question is : what kind of Flow Direction is this? ([16]West? or [4]South)

Beside of the raster data model, I have also made he square and hexagonal Flow Direction model as you can see below.

I hope you can help me to figure out my question.

## closed as too broad by PolyGeo♦Sep 6 '16 at 8:17

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• I'm not entirely sure, but perhaps this is occurring because you have applied the algorithm to a perfectly round space. Nothing in geography is actually exactly perfectly round, and thus the algorithm works. Does this make sense to anyone else? – Sean Adkins Feb 11 '15 at 2:17
• I would expect a perfectly flat surface to be all flow direction 0 (no flow) except if you use 'force' to force the flow, then it will try to do something. The flow direction uses the elevation of the surrounding 8 cells to approximate which way is 'down' for that cell - on a flat surface there is no up or down. – Michael Stimson Feb 11 '15 at 2:25
• Usually flat result in one-directional flow. From help:•If a cell has the same change in z-value in multiple directions and is not part of a sink, the flow direction is assigned with a lookup table defining the most likely direction. See Greenlee (1987). – FelixIP Feb 11 '15 at 2:54