[R-sig-Geo] spTransform, gridded

[Roger Bivand]

On Sat, 24 Mar 2007, Patrick Giraudoux wrote:

> Dear Listers,
> 
> I am trying to convert a Gtiff object  from a coordinate system to 
> another... I have tried the following:

This is warping, so in general the output will not be a regular grid. If 
you need a grid, try interpolating to an empty grid in the target 
projection (IDW in gstat ought to be OK if the resolutions are similar, 
with a small number of neighbours). My guess is that fcLIIe is a 
SpatialPoints* object, not on a regular grid.

You could use gdalwarp externally to do this, if you prefer.

Roger

> 
> # read the Gtiff file (European Lambert azimuthal equal area)
> fcE<-readGDAL("fclambert.tif")
> proj4string(fcE) # CRS OK
> 
> # read a shapefile to get the projection system (Lambert II etendu - a 
> French projection Lambert
> # conformal conic based on NTF)
> fccoms<-readOGR("U:/Documents and Settings/giraudoux/Mes documents/Map 
> tools/Shapefiles checked prj","FC_communes")
> proj4string(fccoms) # CRS OK
> 
> # transform
> fcLIIe<-spTransform(fcE, CRS(proj4string(fccoms))) # looks OK
> 
> # try writing
> writeGDAL(fcLIIe,"fcLIIe.tif")
> Error: gridded(dataset) is not TRUE
> 
> # try to promote to a grid...
> gridded(fcLIIe)<-TRUE
> suggested tolerance minimum: 0.5Error in points2grid(points, tolerance) 
> : dimension 1 : coordinate intervals are not constant
> 
> I  suspect that  pixel centers may have been distorted with the 
> transformation but may naively I wonder if there is a way to turn this 
> problem eg increasing the tolerance...
> 
> Any hint appreciated...
> 
> Patrick
> 
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no