Monday, September 5, 2016

Maps lie, LiDAR doesn't

I was recently on a bike adventure that turned even more adventurous than I expected when a Google Maps mapping error led me and my buddy down a trail that devolved into nothing.  The route was supposed to be the Corral Mainline, a major gravel logging road, but what Google mapped in this portion was actually the route of a natural gas pipeline and not a road at all.  But Open Street Map got it right, as displayed in their OSM Cycle layer:

Two lucky factors let us navigate onward without having to painfully backtrack:

  1. Although we were out of mobile network range, my Google Maps app on my phone had cached some Terrain View tiles.  (This is luck, unlike the regular base map layer, you can't intentionally download Terrain layer maps)
  2. In many areas Google's Terrain View derives its images low-resolution topographic maps, but in parts of Oregon and a few other places it uses LiDAR, a high-resolution elevation mapping technique.  The LiDAR image (on the left above) revealed the topography of the real Corral Mainline so we were able to bushwhack up to it and get back on track. 
Even in Oregon LiDAR coverage is spotty: on our ride we also visited the Nehalem Divide Tunnel but LiDAR was not available there and the ordinary topo maps did nothing to reveal the old railroad bed.