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I am trying to return the area of each polygon (of a multipolygon) which is not overlapping another. When looking at the image below, I would like to return a polygon of just the red part (left side of image), with the blue part removed.

I understand that I could find the intersection using gIntersect or raster::intersect, but the problem is that these circles are part of a single SpatialPolygonDataFrame. Disaggregating the circles is not really an option, I don't think, because my SpatialPolygonDataFrame contains more than 18,000 polygons.

Overlapping Circles

I've provided a sample of reproducible code below (adapted from Cotton.Rockwood on this post. Thank you!

box <- readWKT("POLYGON((-180 90, 180 90, 180 -90, -180 -90, -180 90))")
proj4string(box) <- CRS("+proj=cea +datum=WGS84")
set.seed(1)
pts <- spsample(box, n=10, type="random")
pols <- gBuffer(pts, byid=TRUE, width=50) # create circle polys around each point
merge = sample(1:40, 10, replace = T) # create vector of 100 rand #s between 0-40 to merge pols on

Sp.df1 <- gUnionCascaded(pols, id = merge) # combine polygons with the same 'merge' value
# create SPDF using polygons and randomly assigning 1 or 2 to each in the @data df
Sp.df <- SpatialPolygonsDataFrame(Sp.df1,
                                  data.frame(z = factor(sample(1:2, length(Sp.df1), replace = TRUE)),
                                             row.names= unique(merge)))
plot(Sp.df, col='blue')
i <- gDifference(Sp.df, Sp.df, byid=T)
plot(i, col='red')
Anthony Martinez
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  • Maybe sure to include relevant code in the question itself. It's easier to help if you provide a [reproducible example](https://stackoverflow.com/questions/5963269/how-to-make-a-great-r-reproducible-example) with sample input data so that possible solutions can be tested. – MrFlick Aug 22 '17 at 20:35
  • Thanks for the reminder @MrFlick! I referenced that I added, but did not actually add it. It's taken care of now. – Anthony Martinez Aug 22 '17 at 20:40
  • How do you know that it is two overlapping circles and not three separate shapes? – G5W Aug 22 '17 at 20:54
  • @G5W Because of the way that the data was created. It is a series of polygons created by buffering points. – Anthony Martinez Aug 22 '17 at 21:04

0 Answers0