Hello All:

I wanted to let y'all know that the previously posted "888578-AppalachianTrailCenterline.kmz" and "21124-Appalachian Trail Shelters.kml" files are inaccurate because the coordinate data uses the NAD27 datum, whereas Google Earth requires WGS84 datum.

I've been working extensively with this data since some friends are hiking the entire AT in 2008 and I'll be joining them for a section. I decided to translate the AT centerline data into something I could use in my GPS unit. Man-oh-man did that open up a can of worms! After much work and a lot of pain, this is what I discovered:

The original data used to create the AT centerline file and the AT shelters file came from the AT conservancy website, from their GPS and GIS data page. That webpage clearly states that the data is NAD27. Apparently when these data were converted into .kmz and .kml files the coordinates were not translated to WGS84 as is required by Google Earth.

Because the wrong datum is used, the coordinates are offset to the west and slightly to the south. You can see this effect by loading the "Appalachian Trail Datum Comparison.kmz" file (in a follow-on post) into Google Earth. Take a look at the Description in the Properties page for more information. Briefly, the red NAD27 track (from "888578-AppalachianTrailCenterline.kmz") is displayed west of the green WGS84 track (from attached "Appalachian Trail.kmz") and the green track clearly lines up with the AT whereas the red track does not. Also the shelter waypoints are clearly offset west in the NAD27 data and miss the actual shelters whereas the WGS84 waypoints land on the shelters - or at least closer :-).

Attached is an accurate AT centerline file "Appalachian Trail.kmz" and, in a follow-on post, is an accurate collection of shelter waypoints in "Appalachian Trail Shelters.kmz". In addition to being accurate, these files are more usable since the AT centerline has only 39 track sections (plus 2 short spur tracks to the top of Mt. Washington) whereas the original centerline data had 4549 track fragments. And the shelter waypoints have complete and accurate comments in their Descriptions whereas the original shelter waypoints had less information that was sometimes truncated and incomplete.

I've also created a bunch of files that can be used with a GPS unit (i.e., .gdb and .gpx files). These files show the entire AT or sections of the AT, with varying distances between track points (therefore varying numbers of total track points). Also these GPS files have waypoints for the AT shelters with either a full comment with full information (like the comments in the Google Earth file) or with a brief comment with essential information (e.g. shelter capacity) such that the comment will not get truncated when loaded onto a GPS unit. I plan to make these GPS files available somewhere - haven't decided where yet but perhaps on my nascent website guymott.com - please look there sometime in the near future and I'll have more info available.

I hope all this info is useful.

Guy