[gdal-dev] pixel/line mapping for two images.

Christopher Barker Chris.Barker at noaa.gov
Thu Sep 17 16:52:24 EDT 2009

Belaid MOA wrote:
> I wrote a GDAL C++ code that solves the following simple problem:
> take two images, a source image with spatial resolution 25mx25m (high 
> resolution) and a destination image with spatial resolution 
> 10000mx10000m (low resolution) and scale the source image so that the 
> values of the pixels in the source image that are mapped to the same 
> pixel P in the destination image sum up to the value of P.

you really want the sum, rather than an average?

Anyway, it seems you could use gdal_warp for this, with maybe a 
multiplication at the end to get a sum rather than an average.

 > As a side
> note, the two images have different projections.

That's more than a side note -- this is trivial if there were no 
projection change. Could you do them in separate steps? Change the 
projection of the high-res image, then do your down sampling. That might 
make it pretty easy to do with the gdal utilities.

> Briefly, the arithmetic that computes the overlap is: 

given your large difference in resolution, you may be able to keep it 
simpler -- only use those pixels in the source wholly contained in the 
destination pixel -- I doubt you'd see a difference.

>  As you can see, the ratio 1/2*(SrcXRes/DstXRes) is so small that I do 
> not get smooth image

That's a big change in resolution if your source data is not that 
smooth, you're not going to get a smooth result.

> Am I doing something wrong in my arithmetic?

not that I could quickly see, but give yourself some really simple test 
cases to make sure.

You also might want to test without the re-projection, as that will be 
easier to see if you've done it right.

> Should I apply an average filter to smoothen the image?

That is entirely dependent on your application.


Christopher Barker, Ph.D.

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