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Newbie here. I've downloaded several DEM files from GeoCommunity which correspond to adjacent regions. When I join them together to form a larger relief map there are triangular 'gaps' on all four sides of each 'tile'. It turns out this is because the elevations are all the same in these triangular regions. Visually, it looks like trimming the triangular regions would allow the remainder to fit properly.

Is this because the regions are not in fact rectangular? Something to do with UTM projection? If so how do you know which areas to trim?

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This page might explain it, as well as offer a solution. –  whuber Mar 19 '13 at 3:24
    
Thanks this explains warping is my problem and clearly I need to mosaic the files using third party software. Is there a reliable free package (and ideally one that runs on Mac!) I now know why the literature refers to quadrangles! –  Robert Houston Mar 19 '13 at 14:08
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