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I want to reproject "South polar stereographic" projection to "gcs WGS 84". I have tried the solution given to Re-projecting from Polar projection to EPSG:4326?

through this commands I am getting output in format of WGS84. Moreover latitude value is also correct but longitude values are surprisingly different from expected. what are all possible reasons for this? Code is provided below

gdal_translate -a_srs "+proj=stere +lat_0=-90 +lat_ts=-60 +lon_0=100 +k=1 +x_0=0 +y_0=0 +a=6371200 +b=6371200 +units=m +no_defs" -a_ullr 3818607 -4437313  -4944393  4579687 -of VRT input.tif temp'.vrt
gdalwarp -overwrite -t_srs EPSG:4326 -te -180 -90 180 90 temp.vrt projected.tif
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According to spatilreference.org, your proj4 string is not correct. -a_srs defines the projection but does not modify the coordinates, so your output vrt file is likely to be incorrect (based on a sphere and not an ellipsoid, longitude of origin is 100 and intersection at parallel -70 instead of -60). Below is the proj4 string for EPSG:3412

+proj=stere +lat_0=-90 +lat_ts=-70 +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs

If you are sure that your input data is in NSIDC Sea Ice Polar Stereographic South, you don't need the first line of code and you could write :

gdalwarp -overwrite -s_srs EPSG:3412 -t_srs EPSG:4326 -te -180 -90 180 90 input.tif projected.tif

EDIT: 1) in the proj4 string, you can read the parameters of the projection:

lat_0 is the latitude of origin, that is the summit of the cone in case of stereographic projection

lat_ts is the latitude of the parallel where the cone intersects the ellipsoid

lon_0 is the longitude of the origin of the projection

k is a scale factor

x_0 and y_0 are the shifts of the origin of the cartesian coordinate system (it is used to avoid negative coordinates with some projection)

a and b are the size (in m) of the semi-major axes of the ellipsoid (when they are the same, you have a sphere)

2) Your output pixel size will be in degrees and you can set it using -tr. An average pixel size could be your original pixel size in meter divided by 110000 (this is roughly the size in meter of one degree along a meridian). However, stereographic projection and WGS84 coordinates are very different. Therefore there will therefore be a large amount of resampling due to distortions, and you might have to optimize your pixel size for the location where your work is focused.

  • Thank you. It is working but resolution has changed after reprojection. How to keep it the same ? and I don't understand what is "a" and "b" values here in proj4 string? can you please explain that ? @radouxju – vigna purohit Apr 4 '18 at 16:54
  • I got it what is "a" and "b" here. For resolution I had already tried -tr but it is giving error. – vigna purohit Apr 4 '18 at 17:10
  • What is the resolution now, what value did you use for -tr, and what was the error? – user30184 Apr 4 '18 at 18:16
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    You can't keep the resolution the same because the cell size is now expressed in degrees rather than metres. – mkennedy Apr 4 '18 at 20:05

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