R help - Jul 2012 - precision warning in delaunayn function

Dear R helpers,

I try to use the 'delaunayn' function in the 'geometry' package
for
Delaunay triangulation in 2 dimensions.

For the four following points, I get a warning message :

 > coord=matrix(ncol=2,byrow=TRUE,c(622633,7073452,
+                                  621228,7073517,
+                                  621879,7071762,
+                                  621065,7073331))
 > library(geometry)
 >
 > test=delaunayn(coord,options="Qbb")

qhull precision warning:

The initial hull is narrow (cosine of min. angle is 0.9999999999999999).
Is the input lower dimensional (e.g., on a plane in 3-d)?  Qhull may
produce a wide facet.  Options 'QbB' (scale to unit box) or
'Qbb' (scale
last coordinate) may remove this warning.  Use 'Pp' to skip this 
warning. See 'Limitations' in qh-impre.htm.

I read carefully qh-impre.htm and I still do not understand why I get 
this message.  A simple call to plot(coord) shows that the points are 
not aligned, nor very close.

What did I miss?

Thanking you in advance,

Jean-Luc Dupouey
INRA-Lorrain University
Forest Ecology & Ecophysiology Unit
F-54280 Champenoux
France
mail: dupouey at nancy.inra.fr

Does

 > delaunayn(scale(coord),options="Qbb")

help?

Keith J

On 26/07/2012 08:26, Jean-Luc Dupouey wrote:
> Dear R helpers,
>
> I try to use the 'delaunayn' function in the 'geometry'
package for
> Delaunay triangulation in 2 dimensions.
>
> For the four following points, I get a warning message :
>
>  > coord=matrix(ncol=2,byrow=TRUE,c(622633,7073452,
> + 621228,7073517,
> + 621879,7071762,
> + 621065,7073331))
>  > library(geometry)
>  >
>  > test=delaunayn(coord,options="Qbb")
>
> qhull precision warning:
>
> The initial hull is narrow (cosine of min. angle is 0.9999999999999999).
> Is the input lower dimensional (e.g., on a plane in 3-d)? Qhull may
> produce a wide facet. Options 'QbB' (scale to unit box) or
'Qbb' (scale
> last coordinate) may remove this warning. Use 'Pp' to skip this
warning.
> See 'Limitations' in qh-impre.htm.
>
> I read carefully qh-impre.htm and I still do not understand why I get
> this message. A simple call to plot(coord) shows that the points are not
> aligned, nor very close.
>
> What did I miss?
>
> Thanking you in advance,
>
> Jean-Luc Dupouey
> INRA-Lorrain University
> Forest Ecology & Ecophysiology Unit
> F-54280 Champenoux
> France
> mail: dupouey at nancy.inra.fr
>

Not answering your question but .....

     coord <- as.data.frame(coord)
     names(coord) <- c("x","y")
     require(deldir)
     tryit <- deldir(coord)
     plot(tryit)

yields the attached graph.  No problema.

     cheers,

             Rolf Turner

On 26/07/12 19:26, Jean-Luc Dupouey wrote:
> Dear R helpers,
>
> I try to use the 'delaunayn' function in the 'geometry'
package for
> Delaunay triangulation in 2 dimensions.
>
> For the four following points, I get a warning message :
>
> > coord=matrix(ncol=2,byrow=TRUE,c(622633,7073452,
> +                                  621228,7073517,
> +                                  621879,7071762,
> +                                  621065,7073331))
> > library(geometry)
> >
> > test=delaunayn(coord,options="Qbb")
>
> qhull precision warning:
>
> The initial hull is narrow (cosine of min. angle is 0.9999999999999999).
> Is the input lower dimensional (e.g., on a plane in 3-d)?  Qhull may
> produce a wide facet.  Options 'QbB' (scale to unit box) or
'Qbb' (scale
> last coordinate) may remove this warning.  Use 'Pp' to skip this 
> warning. See 'Limitations' in qh-impre.htm.
>
> I read carefully qh-impre.htm and I still do not understand why I get 
> this message.  A simple call to plot(coord) shows that the points are 
> not aligned, nor very close.
>
> What did I miss?