Originally Posted by
pdlamb
2. GPS plus topographic maps. Sometimes fairly decent, sometimes laughable. My favorite was a climb where the mapping software tried to interpolate between contour lines, with the result that a 1,200' climb registered as 2,700'. The road followed an old logging railroad line, so it was a consistent 2-3% grade most of the way. The elevation plot that thing put out looked more like the logging saw than the railroad elevation.
Originally Posted by
njkayaker
Whatever was being done in this case, it doesn't sound like interpolation.
It's interpolation of elevation from interpolation of two-dimensional (i.e., xy) location. This may be way off topic, but it's at least tangentially related as it explains why mapping solutions are sometimes erratic.
The program figures out where the road is going, and follows the road path. For every interval (for example, every 50 feet of road distance), you figure out the lat/lon or x/y location of the road. Then you figure out, for that point, what's the elevation. Say the road is climbing the side of a mountain, and goes over a gully. The topo lines are the parentheses in this ASCII art, and the vertical bar is the line, and downhill is to the left:
)|)
At the bottom of that segment, the road is halfway between two topo lines (call them 1150' and 1200'). The mapping program will calculate the elevation to be 1175'. Halfway up that segment, the program calculates that the road is 20% of the way from the 1150' elevation to the 1200' elevation, so it says the road dropped to 1160'. Up at the top of the segment, you're back up to 1175', halfway between the two contour lines. You may have actually climbed 5' in 100' of road travel, but the simplistic mapping program will calculate you've descended 15' and ascended 15' -- three times your actual climb!
You can see that you'll get a similar over-calculation if you through a cut around the edge of a ridge. ASCII art for this is: (|(
Better programming will filter these results - most of the time. I suspect some of the programs and web sites have inserted highway department data to correct many widely traveled roads. Still, I treat mapping programs' calculations with suspicion.