Extending a polygon outwards?

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06-21-2012 02:03 PM
SanwaraneeKarunaratne
New Contributor
Hi everyone

I was wondering if there was a way to extend a polygon outwards?
What I have is a polygon of a country with 12 different regions within it and the edge (coast) is lined with these various regions (no region overlaps). I wish to extend these polygons 30m past the coastline. However I can't use a buffer(with the outside only option) because then the regions overlap and it's impossible to clip overlaps when at different points of the coastline, I have different regions overlapping. I also can't use the edit vertices option because there are so many vertices that I would have to individually click and drag outwards to the same distance. Is there a function that can take my polygon and extend the polygon outwards over the coast?

Thanks
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5 Replies
DarrenWiens2
MVP Honored Contributor
You can try:
1.) Trace the coast line to a polyline feature class
2.) Buffer the coast line using "LEFT" or "RIGHT" (requires ArcInfo, or Advanced, now)
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SanwaraneeKarunaratne
New Contributor
Hi Darren

Thanks for your reply-- but I want points that stray 30m away from the coastline to be joined with the same characteristics as the region it's closest to without using a spatial join (because I tried this and it took more than week to process due to the complexity of the polygon) and without a single point snapping to more than one region-- would trace still work?

Thanks
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DarrenWiens2
MVP Honored Contributor
Oh, I think I see. When you tried Spatial Join, did you set a search distance? That can make a big difference. Something else you could try is running Near to find the closest polygon to each point, then join them together (not sure if that would be any faster). You could also make a simplified version of your coastline polygons and try Spatial Join to that.
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SanwaraneeKarunaratne
New Contributor
Yes I had set a search distance of 30m and tried to do the join to the closest region but the processing time wasn't practical.
Ordinarily simplifying would be great but I have to keep the level of accuracy high for this particular project.
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KirstenLuke
New Contributor

I need to do this exact thing, too, for all counties along the US Atlantic Coast. Any other suggestions?  I need  this so I can use a county layer for zonal summaries...my rasters mostly extend slightly offshore to the coast boundary.

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