Hi Gerd, thanks for option. I have prepared a set for tests: http://files.mkgmap.org.uk/download/407/interpolation.7z This set creates maps with different resolutions and interpolations. I have created a direct route and compared height profiles created with different maps. Here my observations: - I can't see any difference between bicubic and bicubic-spline. - Oversampling with DEM resolution twice the resolution of HGT improve look of height profile. It improves shading too. - I have looked at maximum and minimum height of profile. I noticed differences in range 2-6m between bilinear and bicubic interpolation for 3" HGT. My suggestion: use bicubic version. If size is not a problem, try some oversampling. -- Best regards, Andrzej