I am trying to convert a Geotiff from WGS84 to UTM17. In QGIS, there seems to be a super easy way to do this:

First, I input my tiff by clicking add raster layer. This works successfully, as I can see my image on the screen and if I move my mouse of the screen it tells me the GPS coordinates in lat long for every point.

Next, I click save as, and save it as a geotiff with the coordinate system changed to UTM. This appears to work, but it takes pretty long so I have a couple questions.

Originally, the files are about 250mb. When I convert them to another coordinate system, it becomes like 2GB?? Why is it becoming so massive? I would think they should be of similar size.

It takes about 15 min to convert the file while the "save as raster" screen continues. I tried compressing them as JPEG, but that didn't seem to speed things up. I know this is long because I was converting .las point cloud from wgs to utm in Python and it only takes about a minute for the same size.

Am I doing something wrong?


AndreJ's answer is what I was looking for. This is the correct way to change the files. The file size remains the same and it processed 600mb in about 3 minutes.

  • Please edit the question to specify the compression properties of the source and destination images. Projecting an image can involve rotation which can increase file size by a factor of four. Adding in a less efficient (or nonexistent) compression could easily see an order of magnitude files size increase.. – Vince Apr 5 '16 at 21:00
  • For evaluating how heavy an image is to process a better measure than file size is to calculate (width x height x band count x bit depth). Compressed image may be small but every pixel in it must be uncompressed for processing anyways. Fifteen minutes for 2 GB uncompressed image feels still a bit too much. – user30184 Apr 5 '16 at 21:44
  • Hey Vince, I'm not exactly sure I know the compression properties. It was a Geotiff, .tif, that was in a .zip folder originally and then I extracted it before reading it into QGIS. The zipped file was 125mb, then it became 250mb. Where can I find the specific properties? Also, I don't see why rotation, in this case, would increase the size. Since they are georeferenced, wouldn't it stay in the same orientation?? – Walter Morawa Apr 6 '16 at 0:11
  • A projected CRS can be 'tilted' compared to a raster in lat/lon (actually plate carree) or a cylindrical projCRS like Mercator. – mkennedy Apr 6 '16 at 20:10

While Save As ... works easily with vector data, it is not useful to do raster reprojections with it.

Instead, use Raster -> Projections -> Warp for reprojection, or Raster -> Convert -> Translate ... for a different file format.

For large files, it might be better to use the OSGEO4W Command Shell. You can use gdalwarp or gdal_translate with the same parameters as shown in the Raster Menu forms. This way you get the RAM that is otherwise occupied by the QGIS GUI as well.

  • AndreJ you are the best!!!! This works faster and without increasing the file size! – Walter Morawa Apr 7 '16 at 22:14

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