watershed

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06-19-2015 05:41 AM
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mahmodtarazi
New Contributor

i'm using arcGIS 10.1 ,i'm  working on a watershed to find the water path ,i arrives to flow accumulation ,in this step i should have

the path visible but this is what i got

Untitled.png   

how to make the water path clear ??

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9 Replies
IanMurray
Frequent Contributor

As a raster, this is not surprising, especially if you have a very high resolution DEM.  Each flow path is only 1 pixel wide, which doesn't show up at high zoom levels.  Once you zoom in, the flow paths look more correct.

For better symbolizing of your stream, you need to use a raster calculator, to set the minimum number of pixels to be considered a stream, then you can use the Stream to Feature Tool to Convert them to a polyline feature class.

ArcGIS Help 10.1 Deriving runoff characteristics

ArcGIS Desktop Stream to Feature

SepheFox
Frequent Contributor

Yes, this looks like a normal flow accumulation result. As Ian says, it's just that you are zoomed out to 1:600K. Try using the zoom tool to zoom in to those lighter looking pixels on the left of the screen, perhaps 1:40K, and I think you'll see your flow accumulation is fine.

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mahmodtarazi
New Contributor

thank you sephe fox

that's right when you zoom in on the pixels on the lift of the screen i can see the flow accumulation but there must be more flow like in the middle of the screen which i can't find even when i use the zoom in tool

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NeilAyres
MVP Frequent Contributor

Apart rom zooming in to something reasonable given your pixel size, it also helps to play with the raster stretch options in the symbolgy tab. Mine always defaults to percent clip, here changed to standard dev.

FlowAcc_1.jpg

FlowAcc_2.jpg

FlowAcc_3.jpg

mahmodtarazi
New Contributor

thank you for the fast reply ian murry

when i zoom in, nothing changes in the middle of the picture above (which is the area i need)

what i think i need to do is the "raster calculator" you mentioned ,can you help in how to do that??

thank you

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IanMurray
Frequent Contributor

Mahmod,

Take a look at this help thread, it go through all the steps you need for hydrologic analysis.

ArcGIS Help 10.1

The step I described was #6 in that help, where you will need to determine for yourself, what the minimum flow accumulation value is that you want to consider a stream.  You can use the Con(Spatial Analyst) Tool, to determine what values will be considered stream or not stream.  There is a link for the help with the Con Tool in the link I posted, but the goal is to set the minimum flow accumulation values, then turn the flow accumulation raster into a binary raster, either the pixel meets the threshold value and is turned into a 1(so is a stream) or does not meet the threshold value and is a 0 (not a stream).  You may want to test with several values as the threshold until it fits how you feel the watershed should be characterized.

NICOLAPEDDIS
Occasional Contributor II

You need to reclassify the "flowAcc" between 0 and 1 with the raster math and you will se the flow accumulation line "1".

mahmodtarazi
New Contributor

thank you nicola peddis but how i can  do that ?

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NICOLAPEDDIS
Occasional Contributor II

With the raster reclass tool you need to reclassify the low vault to zero and the high vault to 1.

Then with the tool Stream to Feature Tool(or the raster to feature class) you can convert the single pixel 1 to a polyline or polylineM feature class to store also the the length of each portion of the river .

But to understand every single step use the raster math help: It will guide you through the whole the job.

ArcGIS Help 10.1 raster reclass tool

ArcGIS Help 10.1

using reclassification tables