topic Re: Flow Accumulation Generation problem in ArcGIS Spatial Analyst Questions

Flow Accumulation Generation problem

SamHuang — Tue, 25 Jun 2013 16:02:16 GMT

Can someone explain what are the possible problems to why when i run my flow accumulation processing tool, it does not output the flow accumulation correctly. Let me provide you an image to show you what i'm talking about. As you can see within the attachment there is an image which shows the flow accumulation but it does not show entirely of where all the streams are flowing. The circled area are missing for whatever reason and I can't seem to find out what the reason is and how to solve this problem

Thanks in advance!

Re: Flow Accumulation Generation problem

ShaunWalbridge — Tue, 25 Jun 2013 16:19:00 GMT

Try changing the symbology on your raster to include all the values in your range, and you should see the smaller flow accumulation values. Generally, it's most useful to visualize the flow accumulation after thresholding the data, as is being done in the image you attached.

cheers,
Shaun

Re: Flow Accumulation Generation problem

SamHuang — Tue, 25 Jun 2013 16:30:50 GMT

Part of me trying to generate a proper flow accumulation is so that i can eventually generate watershed in which allows me to determine where the upstream and downstream are. But based on this flow accumulation, it ended up having problem with generating the watershed. The result looks like the attached image.

Base on this even if there is a small value, it would not make sense as i know for a fact that stream flow through it, but the accumulation does not show.

Re: Flow Accumulation Generation problem

SamHuang — Tue, 25 Jun 2013 16:34:55 GMT

The reason why I'm asking this is i need to generate a proper flow accumulation so that i can eventually generate watershed in which allows me to determine where the upstream and downstream are. But based on this flow accumulation, it has been causing a lot of problems generating the watershed. The result looks like the attached image. [ATTACH=CONFIG]25472[/ATTACH]

Base on this even if there is a small value, it would not be acceptible as I know for a fact that the stream flow through it, but the accumulation does not show.

Re: Flow Accumulation Generation problem

SamHuang — Tue, 25 Jun 2013 16:36:51 GMT

this is the resulting watershed that i have generated from the accumulation flow. Clearly you can see that it did not generate an upstream or downstream properly

Re: Flow Accumulation Generation problem

JanetBrewster — Tue, 25 Jun 2013 19:32:51 GMT

Maybe you're missing a step? This is what I do to create watersheds:
1) Fill
2) Flow Direction
3) Flow Accumulation
4) Stream Link [via Raster Calculator: "StreamLink (SetNull (FlowAccumulation < n, FlowAccumulation), FlowDirection)", where n is 1% of max flow accumulation but the smaller the number the more stream segments you'll get] -- results in the StreamNetwork raster that I use in step 5
5) Watershed [via Raster Calculator: "Watershed (FlowDirection, StreamNetwork)"]

Janet

Re: Flow Accumulation Generation problem

SamHuang — Tue, 25 Jun 2013 19:53:33 GMT

unfortunately there are no steps missing. I know this for a fact, because there are 70+ watersheds that I have to generate and most of them seem to work with the exception of a few that did not generate properly. It's something that i've been very puzzled about it.

Re: Flow Accumulation Generation problem

ShaunWalbridge — Thu, 27 Jun 2013 04:22:35 GMT

Sam,

Can you provide a small sample of data that contains the issue you're seeing?

cheers,
Shaun