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I'm new to GDAL and map projections, I need to convert equirectangular (EPSG:4326?) projection to "pseudo Mercator" (EPSG:3857?) projection used by 90% web-based maps (which is simple this 0/0/0 file of any OSM tileserver: https://tile.openstreetmap.org/0/0/0.png). I have the following EPSG:4326 example:

ESPG:4326 wold map projection

I run the following commands:

gdal_translate -a_ullr -180 85.05 180 -85.05 -a_srs EPSG:4326 4326.png 4326.tiff
gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3857 4326.tiff 3857.png

But the output (to the left) is obviously not the same as the EPSG:3857 example (to the right):

My result vs EPSG:3857

I feel like I need some offset parameter from the top of the source, or I set wrong a_ullr parameter, or I chose wrong projections. In other words: I need to make a googlemap-like image from the given source. How can I do it in a correct way?

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  • I would first look at making sure your tiff is in 4326. Going from a non-spatial format (png) to a spatial format (tif)
    – GeoMonkey
    Nov 8, 2022 at 20:26
  • How do you know the ullr coordinates of your source image? Where does "EPSG:3857 example (to the right)" come?
    – user30184
    Nov 8, 2022 at 21:33
  • @Nate I thought it's made with gdal_translate?
    – Martin
    Nov 9, 2022 at 4:10
  • @user30184 I'm not sure about ullr parameter, I've looked different sources, spatialreference.org says it should be -180 90 180 -90 but then I get a very strange vertical picture 133x1605px. I've played with it a bit but it seems there's no way to make less space above the Greenland, you can only shift the map center to the north/south. The example of EPSG:3857 "web mercator" comes directly from OSM map, it's the same as many results you can google with this keyword (wikipedia for example).
    – Martin
    Nov 9, 2022 at 4:26
  • The ullr parameters must match with the area that your original image presents. Do you have any metadata about it?
    – user30184
    Nov 9, 2022 at 7:08

1 Answer 1

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I think I've found a solution: so the -a_ullr bounds for the source projection will be -180 90 180 -90, and then I've learned that EPSG:3857 has an extent in coordinates which is -20037508.34 -20037508.34 20037508.34 20037508.34. So the commands will be:

gdal_translate -a_ullr -180 90 180 -90 -a_srs EPSG:4326 4326.png 4326.tiff
gdalwarp -te -20037508.34 -20037508.34 20037508.34 20037508.34 -s_srs EPSG:4326 -t_srs EPSG:3857 4326.tiff 3857.png

And now it looks much better:

enter image description here

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