Message-ID: <dc22b2570905181941u48bdb630v47d47a054a05b669@mail.gmail.com>
Date: 2009-05-19T02:41:52Z
From: Robert J. Hijmans
Subject: rgdal project; inverse does not return original values
In-Reply-To: <dc22b2570905181929u686f3354n4f7910c335a59356@mail.gmail.com>
Sorry, that was stupid,
This:
xyinv <- project(xysin, "+proj=longlat", inv=TRUE)
had to be:
xyinv <- project(xysin, "+proj=sinu", inv=TRUE)
problem solved....
Robert
On Tue, May 19, 2009 at 10:29 AM, Robert Hijmans <r.hijmans at gmail.com> wrote:
> I am confused about this behavior of rgdal
>
>> require(rgdal)
>> # make a matrix of coordinates
>> xygeo <- rbind(c(-180, 0), c(180,0), c(0,0), c(0,20), c(0,90))
>> # project to sinusoidial
>> xysin <- project(xygeo, "+proj=sinu", inv=FALSE)
>> # project back to longlat
>> xyinv <- project(xysin, "+proj=longlat", inv=TRUE)
>> xygeo
> ? ? [,1] [,2]
> [1,] -180 ? ?0
> [2,] ?180 ? ?0
> [3,] ? ?0 ? ?0
> [4,] ? ?0 ? 20
> [5,] ? ?0 ? 90
>> xyinv
> ? ? [,1] ? ? ?[,2]
> [1,] ?180 ? ? ? ? 0
> [2,] -180 ? ? ? ? 0
> [3,] ? ?0 ? ? ? ? 0
> [4,] ? ?0 126759249
> [5,] ? ?0 573070423
>>
>
> Why are rows 4 and 5 different?
>
>
> An illustration of the same problem, using 'raster'
>
>> library(raster)
>> r <- raster(nrow=36, ncol=18, ?projs= "+proj=longlat")
>> xy <- xyFromCell(r,1:ncell(r))
>> par(mfrow=c(1, 3))
>> plot(xy) # ok
>> xysin <- project(xy, "+proj=sinu", inv=FALSE)
>> plot(xysin) ? # nice
>> xyinv <- project(xysin, "+proj=longlat", inv=TRUE)
>> plot(xyinv) ?# ???
>
>
> Robert
>
>> sessionInfo()
> R version 2.9.0 (2009-04-17)
> i386-pc-mingw32
>
> locale:
> LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
> States.1252;LC_MONETARY=English_United
> States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
>
> attached base packages:
> [1] stats ? ? graphics ?grDevices utils ? ? datasets ?methods ? base
>
> other attached packages:
> [1] rgdal_0.6-8 sp_0.9-34
>
> loaded via a namespace (and not attached):
> [1] grid_2.9.0 ? ? ?lattice_0.17-22
>>
>