[R-sig-Geo] doubt with readGDAL

["José M. Blanco Moreno"]

Dear R-users community,
I have been trying to figure out on my own, but no way...
I am trying to read and decimate a geotiff:

library(maptools)
library(spatial)
library(sp)
library(rgdal)

download.file(url='http://srtm.geog.kcl.ac.uk/portal/srtm41/srtm_data_arcascii/srtm_37_04.zip',destfile='srtm_37_04.zip')
shell('unzip -o srtm_37_04.zip')
cata <- 
readGDAL('srtm_37_04.tif',offset=c(2900,0),region.dim=c(2600,5000),output.dim=c(260,500))
cata2 <- cata
cata2 at data$band1 <- ifelse(is.na(cata2 at data$band1),-1,cata2 at data$band1)
cata2 at data$band1 <- 
c(50,100,250,500,1000,1500,2000,3000)[cut(cata at data$band1,c(0,50,100,250,500,1000,1500,2000,3000))]
image(cata2,col=gray(seq(.9,0.6,length=9)))
contour(cata2,add=T,levels=c(0,50,100,250,500,1000,1500,2000,3000),col=gray(.7),drawlabels=F)
axis(1)
axis(2)

But the coordinates don't come out: they are translated in the y-axis *a 
lot*; bbox(cata) returns:
            min        max
x -4.166183e-04   5.000417
y -1.577894e+01 -10.778109

While in the non-decimated version they are:
            min      max
x -0.0004166183  4.16625
y 40.4170835754 42.58375

Could anyone indicate a workaround to preserve the coordinates of the grid?
Thank you for any information on this issue (which I suppose is 
generated by my own incompetence).

José M.

sessionInfo()
R version 2.8.1 (2008-12-22)
i386-pc-mingw32

locale:
LC_COLLATE=Spanish_Spain.1252;LC_CTYPE=Spanish_Spain.1252;LC_MONETARY=Spanish_Spain.1252;LC_NUMERIC=C;LC_TIME=Spanish_Spain.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base    

other attached packages:
[1] rgdal_0.6-8     spatial_7.2-47  maptools_0.7-25 sp_0.9-37       
foreign_0.8-36

loaded via a namespace (and not attached):
[1] grid_2.8.1      lattice_0.17-25


-- 
---------------------------------------
José M. Blanco-Moreno

Dept. de Biologia Vegetal (Botànica)
Facultat de Biologia
Universitat de Barcelona
Av. Diagonal 645
08028 Barcelona
SPAIN
---------------------------------------

http://www.ub.edu/agroecologia/

phone: (+34) 934 039 863
fax: (+34) 934 112 842