I am trying to find a way to apply a rectangular buffer on my spatial polygon dataframe in R. At the end, I would like to be able to shrink my polygons by specified units on both sides. So far, I have tried gBuffer function from rgeos package. However the issue with gbuffer is that it only takes single argument of width and hence applies a uniform buffer on all sides (like buffer by percentage in qgis). My original polygon file has 1800 shapes. Here is a small dummy set to illustrate the problem in R:
library(rgeos)
# simple example, from scratch:
Sr1 = Polygon(cbind(c(1,4,4,1,1),c(1,1,4,4,1)))
Sr2 = Polygon(cbind(c(1,4,4,1,1),c(6,6,9,9,6)))
Srs1 = Polygons(list(Sr1), "s1")
Srs2 = Polygons(list(Sr2), "s2")
SpP = SpatialPolygons(list(Srs1,Srs2), 1:2)
#left panel on the figure
plot(SpP)
plot(gBuffer(SpP, byid=TRUE, width=c(-0.3, -0.3)), add=T)
Here is the output figure:
PS: I am also open to qgis solutions.
