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

[Murray Richardson]

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....
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at stat.math.ethz.ch
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
>