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