A GAP tool I don't know about or one waiting to be invented?
OK. I got the GAP trail book. Full of interesting history with detailed notes by mile. But let's say I'm interested in what's at mile marker 127.4 Round Top Cement Hill (a random example from the book) and want to know just where that is on a map such as perhaps google maps.
I guess someone can make a GPX of KML file with the mile markers. As long as you can access it by url, you can get Google maps to open it. If you have the coordinates of the mile markers I can try to put something together.
IF SCJ SE, Surly LHT, BikeFriday NWT, Cannondale M300, Raleigh 700
Or you would look at the printable map sections available on the web site (e.g., http://www.atatrail.org/tmi/map4.cfm), which have markers every 5 miles, get a rough idea where something is on the map and keep your eyes open for it as you ride. Also, some of the points of interest have displays with info that are pretty hard to miss.