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:
Can anybody help ?
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.
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.