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I'm using the function gUnaryUnion to merge polygons in R. In one particuliar case this doesn't work correctly - some of the old polygon contours are still visible:

enter image description here

Can anybody help ?

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up vote 2 down vote accepted

I suspect your borders don't exactly match up. Either clean your source data or do a little trick I showed on a similar question on StackOverflow - add a little buffer round your polygons, enough to make sure things overlap.

http://stackoverflow.com/questions/17675538/how-to-plot-the-world-without-the-countries-boundaries/17676338#17676338

The exact size of the buffer should be small enough to make sure misaligned edges overlap but not so big as to distort the real boundaries.

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The buffer trick worked perfectly ! Thanks a lot, John –  user2568648 Jul 17 '13 at 10:16
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