[R-sig-Geo] create a triangular grid ?

[Michael Sumner]

On Fri, 30 Sep 2016 at 21:29 Fernando Paim <fernandopaim at paim.pro.br> wrote:

>
>
> Hello everyone,
>
> I need to create a triangular grid for kriging.
> I've tried with makegrid, expand.grid but with just a rectangular grid.
> I have not been able to find a method to trim the rectangular grid to
> the newdata same shape of the original data, is this possible?
>
>
You already have a reasonable triangular mesh here so I assume you want to
generate a rectangular grid (a raster) from this?

d <- read.delim("http://www.paim.pro.br/downloads/dados.csv")

tri <- RTriangle::triangulate(RTriangle::pslg(P = cbind(d$x, d$y), PA =
cbind(d$z)))

rgl::triangles3d(cbind(tri$P, tri$PA)[t(tri$T), ], col = "grey", specular =
"black")
rgl::points3d(d$x, d$y, d$z)

You can use barycentric coordinates to get a fast interpolation for
arbitrary points on this mesh, there are functions for this in geometry
package but you have to drive the subsequent sampling yourself. If that's
on the right track I would generate a regular grid with raster, then
convert to barycentric and pull out interpolated values. I have an old
example here that might help:

https://github.com/mdsumner/xyztrisurf/blob/master/xyztrisurf.Rmd

You can use simple interpolation to get a little more resolution from your
data on the mesh in the first place with a maximum triangle area (the 'a'
argument):

tri <- RTriangle::triangulate(RTriangle::pslg(P = cbind(d$x, d$y), PA =
cbind(d$z)), a = 2)

rgl::triangles3d(cbind(tri$P, tri$PA)[t(tri$T), ], col = "grey", specular =
"black")
rgl::points3d(d$x, d$y, d$z)

These visualizations at least give a sense of what your data are and what
is sensible, the barycentric interpolation to sample points is as if you
intersect those grid points with this surface.

Otherwise you can use real modelling -  depending on what makes sense for
these data . . .

Cheers, Mike.


> The
> data (x,y,z) are available in
> http://www.paim.pro.br/downloads/dados.csv
>
> Can anyone help?
>
> Thanks in
> advance,
>
> Fernando Paim
>
>
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Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
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