[R-sig-Geo] error in trmesh (alphahull package)

Murray Richardson murray.richardson at utoronto.ca
Sat Apr 4 02:55:38 CEST 2009

Hello R community,

I have cross-posted with r-sig-geo as this issue could fall under either 
interest group I believe. 

I just came accross the alphahull package and am very pleased I may not 
need to use CGAL anymore for this purpose.   However, I am having a 
problem computing alpha shapes with my point data, and it seems to have 
to do with the spatial configuration of my points (which form long, 
skinny, contiguous clusters).  If I use rnorm to generate the same 
sample size with the same mean and SD, I can compute the alpha shape 
without trouble (which is why I think it might have to do with the 
spatial arrangement of my points).  See below:

# first here's a quick sample of my data (UTM coordinates)
 > xcoords[1:5]
[1] 670080.2 670080.2 670080.2 670080.2 670080.2
 > ycoords[1:5]
[1] 5005501 5005499 5005498 5005497 5005495
 > xcoords[1:5]
#try the ashape routine with error
 > alpha.shape<-ashape(xcoords,ycoords,15)
Error in tri.mesh(X) : error in trmesh

#get statistics to generate a similar dataset to test against
 > length(xcoords)
[1] 26257
 > length(ycoords)
[1] 26257
 > mean(xcoords)
[1] 670462.4
 > mean(ycoords)
[1] 5005382
 > sd(xcoords)
[1] 149.3114
 > sd(ycoords)
[1] 181.5950

#generate the test data
 > xtest<-rnorm(26257,670462.4,149.3)
 > ytest<-rnorm(26257,5005382,181.60)

# try ashape routine with success
 > alpha.shape<-ashape(xtest,ytest,15)
 > class(alpha.shape)
[1] "ashape"

Thanks for any insight into this!


ps I am able to compute the alpha shapes for this same dataset without 
problem using CGAL but I find it a pain to work with....

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