[R-sig-Geo] Different result from sptransform() and ptransform()

[Agustin Lobo]

Roger,
While the explanation of the error is clear, I am actually not sure
about the practical consequences.
What I understand is that we must not use ptransform() by now (and use
spTransform() instead), and
that ptransform() will be fixed in the future. Am I right?

Thanks

Agus


2012/1/29 Roger Bivand <Roger.Bivand at nhh.no>:
> On Sat, 28 Jan 2012, Agustin Lobo wrote:
>
>> Right, grid1km is the polygon object from whence cents comes from and
>> I copied the wrong line.
>> Sorry for the confusion.
>
>
> The confusion is in proj4 in the ptransform() function, as:
>
>
>> centslonlat<- spTransform(cents,CRS("+init=epsg:4326"))
>> coordinates(centslonlat)[1:3,]
>
>            X        Y
> [1,] 1.315311 42.23956
> [2,] 1.327326 42.24060
> [3,] 1.339342 42.24163
>>
>> ptransform(data = coordinates(cents),
>
>  src.proj = CRSargs(CRS("+init=epsg:3035")),
>  dst.proj = "+proj=longlat +ellps=WGS84 +datum=WGS84"
>  )[1:3,]*57.29577951308232
>         [,1]     [,2] [,3]
> [1,] 1.315311 42.23956    0
> [2,] 1.327326 42.24060    0
> [3,] 1.339342 42.24163    0
>
> ptransform() omits to test for geographical coordinates in the dst.proj=
> argument, and consequently does not multiply by RAD_TO_DEG. The correct code
> is in rgdal/src/projectit.cpp, from line 255. ptransform() is more recent
> than project() in proj4, so the fact that it isn't the same (either source
> or destination as longlat) got overlooked.
>
> Hope this clarifies,
>
> Roger
>