error in trmesh (alphahull package)

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....

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....