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I don't know why does gdalbuildvrt not allow:

  • Unreferenced images
  • Rotated images
  • South up images

Could someone throw some light?

Many thanks in advance

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2 Answers 2

up vote 2 down vote accepted

Because the utilities output is very simple, meant to be a basic mosaic of the easy case for a set of (possibly sparse) tiles. Merging images like this needs some kind of manipulation or warping.

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Ok, thanks for your response. I think I'm going to look for information about merging rotated images. I know it's difficult, but I don't have enough background to argue why. –  jorgeas80 Aug 14 '12 at 15:54
    
As with any warping, it's about generating a new grid of (possibly "rotated") points across the inputs and resampling with them via a given method (nearest neighbour, bilinear interpolation). Plenty of tools can do it to various levels of control, including gdalwarp, which will actually build VRT for you with -of VRT. –  mdsumner Aug 14 '12 at 22:05
    
Thanks for the suggestion! –  jorgeas80 Aug 15 '12 at 10:17
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Well, the answer is that if you want to stack or mosaic a bunch of images, you need to have a common reference frame. It would be quite complicated to mosaic data that are in UTM and data that are in a different projection. You would need to reproject into one of the two reference frames, so that you know where one raster lies with respect to the other.

One way to get around this is to build VRTs of the individual files (it's just an XML file, see the reference for the VRT format). There you can add a GeoTransform (to solve the unreferenced image issue, as well as dealing with rotation using an affine transformation), and if needs be, you may even reproject stuff (or assign projections to them).

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Yes, I'm actually using VRT driver to develop my own one. I've read, in several places, that is hard to work with rotated rasters. Tools like gdal_translate or gdalbuildvrt doesn't work in the input raster(s) is (are) rotated. So, why is so difficult? Isn't enough with the GeoTransform array? –  jorgeas80 Aug 14 '12 at 19:00
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