Mosaic rasters in different CRSes

surtgis mosaic assumes all inputs share a CRS and grid. If they don’t, you’ll hit an error. Two recommended approaches.

Pre-reproject everything to a target CRS

Pick a reference raster (or create an empty one) in your target CRS, then use resample on each input to align them:

for f in tile_*.tif; do
    surtgis resample "$f" "aligned_$f" --reference reference_utm.tif
done
surtgis mosaic aligned_*.tif out.tif

resample handles both the CRS change and the grid alignment in one step, so aligned_*.tif are all on the same grid by construction.

Use stac composite if the sources are on a STAC catalog

The multi-UTM case that drove SurtGIS 0.6.1’s critical fix is exactly this: a bbox spanning two UTM zones, with different scenes in each zone. stac composite handles it automatically by reprojecting each tile’s bbox into the tile’s native CRS before reading, then unifying at mosaic time. You don’t think about zone boundaries:

surtgis stac composite \
    --bbox=-71.5,-35.5,-70.5,-34.5 \   # spans UTM 19S and 18S
    --asset red,nir \
    ...

If you really want to mosaic mixed-CRS files yourself

Reproject them all to a common CRS first (either via resample --reference or gdalwarp), then surtgis mosaic. There is no “mosaic with automatic reprojection” command; deliberately so, because implicit reprojection during mosaic usually produces surprising seams when the target grid isn’t chosen carefully.

Overlap handling

surtgis mosaic uses NaN-aware last-write-wins: a source pixel only overwrites the output if it is finite. This means irregular tile shapes (Sentinel-2 scenes with masked borders, Landsat fill pixels) do not overwrite valid data from adjacent tiles. Order of the input arguments matters only for genuinely overlapping valid regions — there, the last file wins.