[Gdal-dev] projection with GEOS

Tue Jan 30 03:26:36 EST 2007

Hi,

I think you mix up gcp based warping, and reprojection.

If your input image is already in GEOS projection, you shouldn't need
any gcp's. Declare the correct projection, UL corner coordinates and
pixel spacing should suffice. You can do that with gdal_translate
-a_srs "proj=...." and creating an additional .tfw file. For the values
you should put in the tfw file, search on this list or google a bit, I
don't know their order by heart. You can even skip the gdal_translate
a_srs part by using gdalwarp -s_srs <geos projection> -t_srs <wgs84
latlong>.

GCP's are usually used to match *unprojected* imagery (xy pixel space)
to some projection, or even unprojected latlong. Each gcp defines the
pixel x and y (no projection here, just raster indexing) and the
coordinates in geographic space for that pixel. Ideally you then also
should declare the projection system for your gcp points (NB this is
not the projection of your image!), but I don't know if you can do that
with gdal_translate. However, once you have declared your gcp's you
should be able to warp your image to projected space by using gdalwarp
<image> result.tif without any additional arguments. gdalwarp should
detect the gcp's and use them for the warp.

Because your input image is already projected, you probably should not
use gcp's because then the result will be less accurate then when using
the direct reprojection from GEOS to WGS84 latlong. Check if your image
already has reference coordinates and pixel spacing defined with
gdalinfo, if not create a worldfile with the ul coordinates and pixel
spacing, and use gdalwarp -s_srs "proj=geos etc" -t_srs "EPSG:4326"
<infile> <outfile>.

Cheers,
Vincent.

On Mon, 29 Jan 2007 19:29:06 +0100
Hannes Isaak Reuter <hannes.reuter at jrc.it> wrote:

> Hallo Gdals,
> 
> After experimenting around now the second day and not reaching close 
> to a solution, here the question..
> 
> I have a hdf file which is supposed to be in the GEOS projection, 
> which I would like to have in WGS84
> 
> So, what I do is creating a tif with GCPS
> 
> gdal_translate -gcp 1 29 -45,06 76,49 -gcp 1 229 -15,29 53,95 -gcp 1 
> 429 -12,08 43,56 -gcp 1 651 -10,44 34,60 -gcp 101 15 -33,11 78,23 
> -gcp 101 229 -10,17 53,75 -gcp 101 429 -8,08 43,46 -gcp 101 651 -7,00 
> 34,54 -gcp 201 7 -17,42 78,94 -gcp 201 229 -5,21 53,63 -gcp 201 429 
> -4,16 43,40 -gcp 201 651 -3,61 34,50 -gcp 301 3 -1,34 80,71 -gcp 301 
> 229 -0,4 53,59 -gcp 301 429 -0,7 43,37 -gcp 301 651 -0,4 34,49 -gcp 
> 401 6 15,60 79,29 -gcp 401 229 4,53 53,62 -gcp 401 429 3,61 43,39 
> -gcp 401 651 3,14 34,50 -gcp 501 14 29,69 77,92 -gcp 501 229 6,47 
> 53,73 -gcp 501 429 7,53 43,45 -gcp 501 651 6,53 34,53 -gcp 601 27 
> 42,16 76,42 -gcp 601 229 14,56 53,92 -gcp 601 429 11,52 43,54 -gcp 
> 601 651 9,96 34,59 -gcp 701 46 50,92 74,18 -gcp 701 229 19,92 54,20 
> -gcp 701 429 15,61 43,68 -gcp 701 651 13,45 34,67 -gcp 801 71 57,03 
> 71,55 -gcp 801 229 25,69 54,59 -gcp 801 429 19,86 43,87 -gcp 801 651 
> 17,04 34,79 -gcp 901 102 62,17 68,82 -gcp 901 229 32,13 55,12 -gcp 
> 901 429 24,34 44,11 -gcp 901 651 20,75 34,93 -gcp 1001 140 64,66 
> 65,61 -gcp 1001 329 32,86 49,54 -gcp 1001 529 26,68 39,97 -gcp 1001 
> 651 24,62 35,10 -gcp 1101 185 66,80 62,33 -gcp 1101 329 39,32 50,12 
> -gcp 1101 651 28,72 35,32 -gcp 1201 237 69,52 59,09 -gcp 1201 329 
> 47,24 50,93 -gcp 1201 651 33,10 35,58 -gcp 1301 297 72,54 55,79 -gcp 
> 1301 329 58,94 52,27 -gcp 1301 651 37,91 35,90 -gcp 1401 367 74,10 
> 52,10 -gcp 1401 651 43,31 36,29 -gcp 1501 448 75,27 48,16 -gcp 1501 
> 651 49,71 36,81 -gcp 1601 542 76,37 43,94 -gcp 1601 651 58,07 37,54 
> -gcp 1700 651 77,40 39,39 HDF5:"HDF5_FVC200612020000.hdf"://FVC
> testdd.tif
> 
> this seems to work..
> 
> Next step would be to reporject that tif to WGS84 with the following
> line
> 
> gdalwarp -s_srs "+proj=geos  +lon_0=10 +h=35785831m +x_0=0 +y_0=0" 
> -t_srs WGS84 testdd.tif test270106.tif
> 
> However this delivers a tif which is completly out of the data range 
> ( from 9 to 10 degress in x extent)
> 
> If I apply
> 
> gdalwarp -s_srs WGS84  -t_srs WGS84 testdd.tif testutm.tif
> 
> at least the image comes closer to the desired location, still up to 
> 2 degress off.. so no real solution ..
> 
> 
> Last think i tried was to warp the image without GCPs to WGS84, still 
> thats not working .. see below..
> gdalwarp -s_srs "+proj=GEOS  +lon_0=10 +h=35785831km +x_0=0 +y_0=0" 
> -t_srs WGS84 testdd5.tif testdd6.tif
> ERROR 1: Unable to compute a transformation between pixel/line
> and georeferenced coordinates for testdd5.tif.
> There is no affine transformation and no GCPs.
> 
> 
> 
> Can anyone shed a little bit more light on how to reproject GEOS 
> imagery using GDAL ?
> 
> 
> Hannes
> 
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