[R-sig-Geo] spTransform changes the class

[Michael Sumner]

I forgot to mention the first time, spTransform will not reproject a
grid but it will coerce the grid to points and then return reprojected
versions of those. That's why Agustin sees the class change from grid
to points.

You can see this in the method definitions for SpatialGrid/PixelDataFrame, e.g.


setMethod("spTransform", signature("SpatialPixelsDataFrame", "CRS"),
	function(x, CRSobj, ...)
		spTransform(as(x, "SpatialPointsDataFrame"), CRSobj, ...))

setMethod("spTransform", signature("SpatialGridDataFrame", "CRS"),
	function(x, CRSobj, ...)
		spTransform(as(x, "SpatialPixelsDataFrame"), CRSobj, ...))


Cheers, Mike.

On Fri, Jul 2, 2010 at 1:51 AM, isabelle boulangeat
<isabelle.boulangeat at gmail.com> wrote:
> Hi!
>
> Are you sure that the *spTransform *method can be applied to a SpatialGrid
> object? I think it only works for SpatialPoints, SpatialLines and
> SpatialPolygons.
> If you are working with rasters, perhaps you should use the function
> *projectRaster
> *from the package raster.
>
> I hope it helps...
>
> Isa.
>
> 2010/7/1 Agustin Lobo <alobolistas at gmail.com>
>
>> Hi!
>>
>> I use spTransform to reproject:
>>
>> > class(eugrd025)
>> [1] "SpatialGrid"
>> attr(,"package")
>> [1] "sp"
>>
>> > projection(eugrd025)
>> [1] "+proj=longlat +ellps=WGS84"
>>
>> > eugrd025EFDC <- spTransform(eugrd025,
>> CRSobj=CRS(projection(eugrd025EFDC_SPDF)))
>>
>> > projection(eugrd025EFDC)
>> [1] "+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000
>> +ellps=GRS80 +units=m +no_defs"
>>
>> This is good, but:
>> > class(eugrd025EFDC)
>> [1] "SpatialPoints"
>> attr(,"package")
>> [1] "sp"
>>
>> Why is the class changed to SpatialPoints? I need a grid object for
>> > eugrd025EFDCr <- raster(eugrd025EFDC)
>> > Br025 <- resample(Br,eugrd025EFDCr)
>>
>> don't I?
>>
>> I also have the problem that the CRS of eugrd025EFDCr is not conserved:
>> > projection(Br)
>> [1] "+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000
>> +ellps=GRS80 +units=m +no_defs"
>>
>> > projection(eugrd025EFDC)
>> [1] "+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000
>> +ellps=GRS80 +units=m +no_defs"
>>
>> > projection(eugrd025EFDCr)
>> [1] "NA"
>>
>> which I fix with
>> > projection(eugrd025EFDCr) <- CRS(projection(eugrd025EFDC))
>>
>> and repeat
>> > Br025 <- resample(Br,eugrd025EFDCr)
>>
>> Anyway, I get a much coarser raster than I want:
>> > dim(Br025)
>> [1] 10 10  1
>>
>> because
>> > dim(eugrd025EFDCr)
>> [1] 10 10  1
>>
>> While it should be 180x100:
>>
>> > summary(eugrd025EFDC)
>> Object of class SpatialPoints
>> Coordinates:
>>       min     max
>> s1 2498538 6561151
>> s2 1327858 4313263
>> Is projected: TRUE
>> proj4string :
>> [+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=GRS80
>> +units=m +no_defs]
>> Number of points: 18000
>> > dim(Br025)
>> [1] 10 10  1
>>
>> > summary(eugrd025)
>> Object of class SpatialGrid
>> Coordinates:
>>              min    max
>> coords.x1 -10.125 34.875
>> coords.x2  34.875 59.875
>> Is projected: FALSE
>> proj4string : [+proj=longlat +ellps=WGS84]
>> Number of points: 2
>> Grid attributes:
>>  cellcentre.offset cellsize cells.dim
>> 1               -10     0.25       180
>> 2                35     0.25       100
>>
>> Is eugrd025EFDCr wrong because class(eugrd025EFDC) has been changed
>> from SpatialGrid to SpatialPoints?
>>
>> Agus
>>
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>>
>
>
>
> --
> Isabelle Boulangeat, PhD student
> Ecosystem dynamics and plant traits
>
> Bureau 203, +33 (0) 476 635 733
>
> Laboratoire d'écologie Alpine (LECA)
> Grenoble, France
> lab' s website : http://www-leca.ujf-grenoble.fr/
> my website : http://j.boulangeat.free.fr/
>
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