I have modified an existing DEM to produce two sinks - the brightest pixels, both with a dot drawn on them below.

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I have verified that they are, in fact, the lowest points with the ID tool and have confirmed that flow should follow successively lower elevation values to the center point with the "interpolate line" graph.

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I would expect that the flow accumulation result over this area would show the highest value at these low points. In fact that's what I'm trying to force in my model - I want these depression to act like a catcher's mitt for flow lines in the area since my pour points are not perfect.

However the result I get is that the low point shows a flow accumulation of "0" and there are multiple flow lines into each depression that do not connect and do not flow to the low point. all flow lines terminate at the pixel immediately before the low point rather than merging into it

Is this the expected behaviour for this tool?

  • It should be nodata cell followed by fill. – FelixIP Mar 28 '19 at 20:32
  • If I understand, you're suggesting that I should set the sink pixels (to which i want to direct flow) to be NoData? or are you saying that they should be NoData in the Flow Accumulation result? – Zipper1365 Mar 28 '19 at 20:36
  • I explicitly don;t want to fill these, I have pour points at these sinks and need to funnel flow to them. Fill undoes that. no? Perhaps I could fill with a z-limit? – Zipper1365 Mar 28 '19 at 20:41
  • I should set the sink pixels (to which i want to direct flow) to be NoData? Yes. – FelixIP Mar 28 '19 at 20:46
  • I made a quick test grid with a Null value in the middle. the result is still that multiple flow lines terminate at the pixels surrounding the NULL one. only partial flow is captured in "snap pour point" and then "watershed". If I'm asking to the tool to do what it;s not meant to that's fine I guess, but I thought I had a home run with this funnel idea – Zipper1365 Mar 28 '19 at 21:10

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