I'm currently planning a 5-night trip through Kings Canyon in a couple of weeks, and I'm finding a lot of differences in trail mileage estimates among various sources. For example, some of the trail mileages I've found from Road's End to the Woods Creek Crossing (JMT) are:
Nat Geo Map - 13.5
RJ Secor - 15
Falcon Guide and other trail books - 16
nps.gov web site - 17
That's around a 25% difference from lowest to highest. Not a big deal for a 1 or 2 day trip, but could be significant over a 75-mile trek. May even require planning for an extra day. There are many other examples out there as well.
So who do you trust? Nat Geo? NPS? Or just add 'em all up and take an average?