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I need two polygons. I already have one (the river edge boundary as seen below) enter image description here

What i want to do is create another polygon which covers the part of the raster which does not fall inside the river polygon. It will therefore need to snap to the outside of the river polygon which is a complex shape. I have tried the auto-complete polygon tool but not having much luck. Furthermore, i have part of the river polygon which is separate from the rest. This would need to be classed as the river polygon and not outwith. enter image description here

  • What license level are you using. You could create a polygon that matches your raster. Then you could select your use your existing polygon and use the split tool on the Advanced Editing toolbar to cut it nto the "raster" poly, then delete that. If you only have the basic version then you could split the "raster" poly using the trace tool. Or with the Advanced version you could erase your "raster" poly with your river polys. – johns Jul 23 '18 at 16:49
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I would recommend using Raster to Polygon to create a polygon from the raster.

Then you can use Erase to remove the area covered by the river polygon(s). The output will be your desired nonriver-raster polygon. Note: this tool require an advanced license

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    To compliment this answer: if you don't have an advance license then you could union the two datasets and manually delete out the river to achieve the same affect. I'm not sure Raster to Polygon will work here as there is a missing step in which you need to convert your DEM to a masking raster. I discuss using the CON tool to achieve this in this Q&A so have a read there. You already have a single band raster for the DEM so just use the CON tool. – Hornbydd Jul 23 '18 at 21:13

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