Ahhh - so simple, my mistake. Thanks!
Murray
José Manuel Blanco Moreno wrote:
Hello,
The problem is that your first three points CANNOT be collinear. This
is a feature of trmesh in tripack. So: either you "unsort" your data
(keeping the indices you can sort them again) or alter (e.g. jitter)
the coordinates very slightly. Either solution will do the work.
Cheers,
José M. Blanco
Murray Richardson escribió:
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!
Murray
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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