Hi Andrzej, to be honest I did not understand the math, that's why I've copied the code from Frank. Maybe I've done something wrong with the calculation of dx and dy. Do you think that Franks code has the same problem? Gerd ________________________________________ Von: mkgmap-dev <mkgmap-dev-bounces@lists.mkgmap.org.uk> im Auftrag von Andrzej Popowski <popej@poczta.onet.pl> Gesendet: Dienstag, 9. Januar 2018 15:45:14 An: mkgmap-dev@lists.mkgmap.org.uk Betreff: Re: [mkgmap-dev] HGT - getElevation() Hi Gerd, I'm trying to find if resolution can be better. I think current interpolation is not good. Imagine two combination of 4-points heights from HGT with values like: 1 0 0 7 and 0 1 7 0 For a point in the middle of rectangle, current interpolation would give result 0 in first case and 4 in second, while I would expect 2 in both cases. I think the code, which calculates interpolation based on 3 points, should be used only if the forth is void. There could be 3 combinations of triangles in this case. But if all 4 points are valid, there should be applied standard bilinear interpolation. Something like: if (hlb == HGTReader.UNDEF) { return triangle(... } if (hrb == HGTReader.UNDEF) { return triangle(... } if (hlt == HGTReader.UNDEF) { return triangle(... } if (hrt == HGTReader.UNDEF) { return triangle(... } // 4 points valid return bilinear(... -- Best regards, Andrzej _______________________________________________ mkgmap-dev mailing list mkgmap-dev@lists.mkgmap.org.uk http://www.mkgmap.org.uk/mailman/listinfo/mkgmap-dev