I'm making a map of the Colorado Trail and obtained a set of waypoints taken down the entire trail. I projected the set of waypoints from WGS84 (unprojected) to NAD 83 UTM13N after adding to the data frame which is also in the latter projection). Coordinates display as northing/easting meters as expected. Everything looks fine but using Calculate Geometry to compute trail length (total and individual section lengths) returns erroneous underestimates of published lengths (the published lengths have been measured with a distance wheel). Not even close-- off by nearly 100 miles for the whole trail. I computed surface length using a 1/3 arc sec DEM but this in no way accounts for the error. I can't figure this out but I feel it has something to do with the projection and how it relates to the initially unprojected waypoints. Can anyone clear this up as I am out of ideas.