Create land polygons from water polygon layer

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01-30-2013 05:58 AM
LindsayShort
New Contributor II
Hello all,
I believe this is possible but I'm having a hard time trying to figure out how to get it done.  I have a layer that shows all non-inland water, and I would like to create land polygons from the coastline that the water polygon has created.  Which tools do I use to effectively invert the water layer to create the land polygons?

Thanks,

Lindsay
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RichardFairhurst
MVP Honored Contributor
I forgot to mention that you probably would need to use the Explode Mulit-Part Feature tool that you can access from the Advanced Editor toolbar or as a Geoprocessing tool.  That is because the Union tool keeps all parts of an original feature together that it can by default.  If there is an option on the Union tool to not create a multi-part feature, use that option.

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GayathriAlallasundaram
New Contributor III
Hi,

You could use the Trace tool from the Editor toolbar .
1. Start an edit session on the new feature class.
2. Pick the polygon you want to trace using the select tool in the Editor toolbar.
3. Choose the trace tool from the editor toolbar .
4. Trace the polygon.
5. Finish the sketch.
6. Save your edits.

I hope this helps.

Thank You,
Gayathri A.
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LindsayShort
New Contributor II
Thanks for the suggestion, but I'm working with an area with many many islands as well, I was really hoping there was just a tool that could just invert the polygon.  I tried using the Erase tool, but the area has too many line segments to process.
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RichardFairhurst
MVP Honored Contributor
Thanks for the suggestion, but I'm working with an area with many many islands as well, I was really hoping there was just a tool that could just invert the polygon.  I tried using the Erase tool, but the area has too many line segments to process.


Create a separate shape feature class.  In it create a single polygon that covers all of the holes in your water layer.  The shape can be a square.  If you overbound the water do it by a fair amount so it will be easy to tell it falls outside of the water area.  Use the Union tool to merge and intersect the water shape(s) and the overlapping new shape. 

Every output shape in the Union result that has -1 as the water FID is either an island or a shape that fell outside the external boundary of your water shape(s).  Select everything where the water FID >= 0 and delete them.  Or else do the opposite selection (Water FID = -1) and export them.  Clean up any land shapes that overbounded the water body that you do not want.  You should have your islands.
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RichardFairhurst
MVP Honored Contributor
I forgot to mention that you probably would need to use the Explode Mulit-Part Feature tool that you can access from the Advanced Editor toolbar or as a Geoprocessing tool.  That is because the Union tool keeps all parts of an original feature together that it can by default.  If there is an option on the Union tool to not create a multi-part feature, use that option.
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ChrisSnyder
Regular Contributor III
Create a rectangular FC with the same extent (or larger) as the water FC (you can use this tool to do that: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00170000003q000000) . Erase (use the erase tool) the water FC from the rectangular one. Use the MultipartToSinglepart tool if you want all the islands as individual polygons.

You can use this same process to go from land to water.
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