I am using QGIS 2.2 Valmiera to try to find the average slope of a water course. I've drawn in the water course as a polyline over a large number of contour lines (the water course is long with a very large catchment area). What I've tried to do so far is create a tin file. I've used the Raster > interpolation function using the heights attribute of the contour line shapefile to make the tin and each time I try it gives me a mini-dump error. I've tried to simplify the geometry (from 392515 to 66801 vertices) but I still just get the mini-dump error.
Is there another way to get the slope of this polyline without having to make a tin file or am I doing something wrong in trying to make the tin?