I am new in this field and I am having a problem with my DEM, (SRTM 30M).
When I calculate flow accumulation a straight line appears if I fill the DEM. And rivers are not connected if I don't fill it.
How can I solve this problem?
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1Does that straight line appear in a sink? Maybe a lake or very flat and low area? I guess you mean the straight line between the large brown and green patches? – PythonStudent Apr 21 '17 at 8:03
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yes straight line between green and dark purple – skhan Apr 21 '17 at 10:25
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yes, when I fill sink this straight line appears, what is the solution for it? I can not delineate my watershed due to it – skhan Apr 21 '17 at 10:27
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1Please edit the question in response to requests for clarification. It's not fair to those who would answer to need to mine the comments for critical information. – Vince Apr 21 '17 at 10:43
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1That looks like a flow direction surface and not a flow accumulation surface. When you fill a surface model, those pixels with no connectivity (the sinks) are altered (raised). Any cells that flow into the sink just carry on in the same direction that they entered the sink area. You have to accept this kind of error in the data and just carry on. The sink area will be included in whatever watershed it is bound within. – GBG Apr 27 '17 at 17:33
The issue looks like its due to the nature of the fill algorithm (as others have mentioned), which can create large flat areas with parallel flow lines. Your other option, although its a little more difficult in ArcGIS, is to use a breaching algorithm. Open source GIS software like WhiteboxGAT has tools available to do this.
If you have a previously delineated stream network, you could rasterize and burn this network into your DEM to force flow to that network. You could also burn a flowline into your flat area are then rerun the fill algorithm so that the flow in that area is forced into the drawn flowline.
The line that shows up is probably a normal thing to happen. If it is a flat area all cells from a large area flow into 1 line. This line will get a very high number and will appear as a straight line.
