Message-ID: <49D7B735.3010701@utoronto.ca>
Date: 2009-04-04T19:38:29Z
From: Murray Richardson
Subject: error in trmesh (alphahull package)
In-Reply-To: <49D747CD.5020701@ub.edu>
Ahhh - so simple, my mistake. Thanks!
Murray
Jos? 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,
> Jos? M. Blanco
>
>
> Murray Richardson escribi?:
>> 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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>
>