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When reprojecting a DEM (e.g., SRTM-derived 1-arc DEM) using PCI Geomatica for example, to constrain pixel size and maintain a square pixel you can either change the image extent or the number of pixels, or both. What are the implications of changing either parameters with respect to preserving the DEM accuracy?

  • When projecting from geographic to projected the image will skew and warp, this will be more pronounced for larger images. The cell size (in DD) will not convert exactly to metres and width/height at the top of the image will not be the same as the bottom of the image. I'd change the cell size to slightly larger than the projected average cell size and then maintain some semblance of accuracy using a bilinear or spline resampling method and let the extent be whatever is calculated as there will be some empty pixels due to warping. – Michael Stimson Feb 2 '15 at 0:00
  • Many thanks Michael, this confirmed my thoughts, particularly concerning resampling methods. Bilinear would be the option of choice (for PCI), as cubic-convolution appears to alter the DEM more than I feel comfortable with. Regarding pixel size, 30 m grid spacing is required, which is slightly smaller than 1-arc second, or ~30.8m at the latitude of my project (9°). If I select 'maximum bounds', the number of col/lines will be greater, logically. If I don't select 'maximum bounds', than the 'lower' corner locations provide a smaller footprint, which I found puzzling. Any thoughts on that? Pierr – Pierre Dubeau Feb 6 '15 at 0:39
  • 30.8 (approx) is close enough to 30 metres. Can you put in a picture and details of the difference in footprint, I suspect the only difference is how much NoData appears around the image and the 'value cells' are the same. Cubic can be a little over enthusiastic especially where values change a lot over a little distance - bilinear would be my choice too. Would you consider gdal.org/gdalwarp.html with -tr 30 30 instead? – Michael Stimson Feb 6 '15 at 0:53

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