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I have downloaded SRTM elevation tiles for England and Wales from USGS EarthExplorer. There are around 40 raster files to cover England and Wales. I now need to use QGIS to create slope and aspect measurements. Should I merge the tiles first and then reproject to EPSG:27700 (OSGB36 / British National Grid) or reproject each tile first and then merge. I have tried both methods and the differences are sometimes large.

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  • I would merge tiles into a GDAL virtual raster (VRT) and reproject from the VRT. That helps avoiding artifacts at tile boundaries. If tiles are reprojected individually then I would recommend to run gdalwarp with "target aligned pixels" -tap option and manually selected pixel size -tr.
    – user30184
    Jan 2 at 18:33
  • Thanks, that's very helpful Jan 3 at 11:04
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    Consider putting the initial comment as an answer? Jan 15 at 21:11

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In most cases you should merge first and then reproject. Otherwise you tend to end up with gaps and overlaps at the tile boundaries.

You can either use the standard Merge tool, or build a VRT (virtual raster) and then reproject. The VRT is usually a better option if you want to re-use the merge for different purposes (eg clipping, or reprojecting to a different CRS), as it takes up minimal space.

If you try to reproject tiles individually, there are options in gdalwarp to align pixels to the original grid (-tap with -tr as the original grid size). However, the edges of the extents are still likely to have artifacts when you merge. I'd avoid this if I were you.

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