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This question already has an answer here:

Let us consider a set S consisting of a collection of grid locations (i,j),i,j=1,\ldots,n, in 2-dimensional gridded plane. Here the point (i,j) corresponds to a rectangular grid in the X-Y plane.

This set has the property that one can reach any element (i,j)\in S from any other element ({i'},{j'})\in S through a series of neighbouring grid points which are also in S.

I am interested about the shape of the set S in the plane, for this reason I need the boundary grid locations of the set S.

enter image description here

Note: R-code

data=read.csv(file="trial.csv")

require('ggplot2')

pred.points <- ggplot(data = data,
                      aes(x = lat,
                          y = lon,
                          colour = variable)) + 
  geom_point()

print(pred.points) 

I need an algorithm to find the boundary grid locations for the set?

Here I don't know whether the set is concave or convex.

marked as duplicate by whuber Nov 8 '14 at 16:57

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • Can you add an image illustrating your issue? – SS_Rebelious Nov 5 '14 at 8:31
  • So your primary tool is R? Consider adding your code then so we could understand what exactly have been plotted here. – SS_Rebelious Nov 5 '14 at 10:42
  • Yes I used R. I edited the figure and added the R-code. – Janak Nov 5 '14 at 11:26
  • 1
    You might want to look into alpha hulls. There exists an R implementation in alphahull::ashape. – whuber Nov 5 '14 at 19:29
  • Thank you for letting us know. To help connect future readers to an answer, I will close this as a duplicate of a question whose answers include the R solution (along with solutions on other platforms). – whuber Nov 8 '14 at 16:57