Does anyone know whether it is possible to measure from an actual plotted route distance gpx file compared to the straight line distance to take into account ascent and descent?
Yes, it's possible, but the accuracy would depend a lot on the quality and granularity of the height data you have.
So in our GPS mapping app, where we do calculate total ascent and descent for a track2 or a route1, we use height data from OS (in Great Britain) and NASA (in Ireland). The granularity of this in GB is every 50m (and it's a bit worse in Ireland), so we can give a reasonable approximation for ascent and descent but the real accuracy depends a lot on how smooth the ground is.
For example if the height at two points 50m apart is 1m different we would record 1m of ascent (or descent as appropriate). However the distance walked could be as little as √2501 (with a little help from Pythagorus!) or 50.01m, assuming a steady rise over that 50m, or rather more if the terrain over that 50m went up and down significantly ... which is why granularity is so important here.
But perhaps when we've got access to the next generation height data from LIDAR surveys this might be something people will try to do ...
Hi Paul, Many thanks for your take on this.
After some exploration, I have managed to compare a surface line distance of a 3.5 km track plotted via Global Mapper from a 1M DTM LiDAR file, with the same distance plotted using the Walklakes GPS mapping facility . Although this is a shortish route and not very hilly I found that the LiDAR approach gave a 10-fold larger difference. So this would confirm what you suggested. I shall now try this on a longer more hilly route to see if this holds true. Regards, John