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I created a GeoTiff raster in python, that is projected with the following CRS: *"+proj=merc +lon_0=34.9586601257 +k=1 +x_0=-0 +y_0=1453756.01544 +a=6370997 +b=6370997 +units=m +no_defs"*

When I load it in QGIS, I set project CRS from layer and enable on the fly CRS transformation. Then I open a shapefile of the country (Tanzania), but it doesn't overlap, even if I set the same CRS manually. I'm sure that the raster extent is correct. It looks that it is a problem from the shp. Here is a sceenshot: It is working if I georeference my raster with decimal degrees WGS84... But not if I use the projection coordinates. That is strange. Screenshot:

I did the same with another raster I also created in python, that is projected with: *"+proj=lcc +lat_1=51.4259986877 +lat_2=51.4259986877 +lat_0=51.5159492493 +lon_0=2.60400009155 +x_0=163358.880084 +y_0=163358.804499 +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs"* And with this raster, I can properly overlap the shapefile of the country!

I don't understand why it is working with a CRS in lambert and not with a CRS in mercator.

Any idea?

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If you could offer a picture, with illustrations of the magnitude of the mis-alignment, it would be helpful. – MappingTomorrow Jun 29 '12 at 15:00
I added links to screenshots. Thanks :) – Greg Jun 29 '12 at 15:23

Sorry, very late answer!

It looks like there is a strictly east-west offset to the data. Could the lon_0 (central meridian) parameter be incorrect? Or you used x_0 (false easting) in the Lambert conformal conic case, perhaps you should use it for the Mercator case too.

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