I was curious to see how well OpenStreetMap (OSM) had these locations mapped: http://www.openstreetmap.org/#map=19/37.77620/-122.42455
https://mc.bbbike.org/mc/?lon=-122.424438&lat=37.776229&zoom...
The bulk of my updates are local trails through the bush from when I go exploring, based on my GPS tracks. But I've also been adding pedestrian paths between streets, new roads from residential developments, and so on.
I'm guessing it just takes a few of this kind of person to keep OSM up-to-date. It's a shame it's not more widely used directly by the general public with some way of flagging errors.