[gdal-dev] Mollweide to plate_carree

[Rahkonen Jukka]

Hi,

I tried to go from a Natural Earth world map in EPSG:4326 into Mollweide this way:
gdalwarp -t_srs "+proj=moll +lon_0=0 +datum=WGS84" HYP_50M_SR_W.tif molltest.tif

The warped image has corner coordinates:
Corner Coordinates:
Upper Left  (-18040095.696, 9020047.848)
Lower Left  (-18040095.696,-9020044.507)
Upper Right (18040095.696, 9020047.848)
Lower Right (18040095.696,-9020044.507)

Warping back to EPSG:4326 looks rather good with some small artifacts at the corners. Thus, the corner coordinates for -ullr should be close to these values. However, your original is wider that the Earth. Clip the margins away so that the globe sits tightly in the box and try with -ullr -18040095.696 9020047.848 18040095.696 -9020044.507.

-Jukka Rahkonen-

Lähettäjä: Rahkonen Jukka
Lähetetty: maanantai 28. lokakuuta 2024 23.21
Vastaanottaja: Rick Kohrs <rick.kohrs at ssec.wisc.edu>; gdal-dev at lists.osgeo.org
Aihe: Re: Mollweide to plate_carree

Hi,

I do not really know how to georeference such Mollweide image, but certainly it is not with -a_ullr -180 90 180 -90. The upper-left and lower-right corners mean the corners of the raster, and both those corners are on the black area outside the Earth. Look at the earth-egg and you can imagine that the coordinates -180 90 180 -90 are at the poles and they are not at all at the corners of the image. But even if you make the poles as ground control points and save them into GeoTIFF with correct pixel coordinates (something like 630x 58y and 630x 660y), you will only have two GCPs on a vertical line and that is not enough for warping. I apologize that I can't tell how to do it right and where to get enough usable GCPs.

You wrote "The results are still in a mollweide projection" but that is not true. You can use gdalinfo and see that the result is in EPSG:4326, but because of using -a_ullr -180 90 180 -90 in the first step, there is no need to move any pixel.

-Jukka Rahkonen-

Lähettäjä: gdal-dev <gdal-dev-bounces at lists.osgeo.org<mailto:gdal-dev-bounces at lists.osgeo.org>> Puolesta Rick Kohrs via gdal-dev
Lähetetty: maanantai 28. lokakuuta 2024 22.28
Vastaanottaja: gdal-dev at lists.osgeo.org<mailto:gdal-dev at lists.osgeo.org>
Aihe: [gdal-dev] Mollweide to plate_carree

I have full earth mollweide projected images:

https://www.ssec.wisc.edu/~rickk/wvsst/latest.jpg

and need to reproject the image into plate_carree

I have the following script:
gdal_translate -of GTiff -a_srs "+proj=moll +lon_0=0 +datum=WGS84" -a_ullr -180 90 180 -90 global-wvsst_24291_1800.jpg intermediate_mollweide_georef.tif
gdal_edit.py -tr 0.1 -0.1 intermediate_mollweide_georef.tif
gdalinfo intermediate_mollweide_georef.tif
gdalwarp -t_srs "EPSG:4326" -r bilinear intermediate_mollweide_georef.tif final_output_plate_carree.tif
gdalinfo final_output_plate_carree.tif

The results are still in a mollweide projection.  Any assistance would be greatly appreciated.

Rick
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