Hi there, 

I am trying to downsample a high-resolution file of gridded population 
estimates (1km res over the whole world). I want the downsampled version to 
give the total population in the new lower-resolution grid. However, whenever I 
downsample using gdalwarp -r sum, I don’t seem to be getting this. Instead the 
new values in the lower-resolution grid (no matter what resolution it is) seem 
to be more like the max, rather than the sum, of the old high-resolution grid. 
If I add up the total population across the whole grid it is far lower than it 
should be. How much lower depends on how much I have downsampled. 

I am making sure to exclude the no data values, and have tried a whole variety 
of different output resolutions and projections. Any suggestions as to why this 
might be? 

Thanks in advance for your help! 
Ainslie
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