When you import a GPX track, via DEM Earth, you may often find that there are not enough points included in the GPX file, to be able to smoothly drape the track onto a a real landscape, or that the resulting spline is too rounded and does not match the straight lines you are expecting. The solution to this problem, is to  convert the spline to linear and add artificial points. You can then force the new points to fit the landscape perfectly, by using The GeoModifier, which comes with DEM Earth. This video shows you how easy this is to resolve.