From: Ben Discoe [mailto:ben@vterrain.org]
Sent: Thursday, February 24, 2005 3:32 AM
Subject: TMK SHP vs. DLG

I've spent a bunch of time comparing the Big Island TMK SHP with the transportation DLG layers. First i had to reproject the TMK - it is UTM zone4, which is a little strange, as the island is really in zone5, all but 2 USGS quads are in zone5.

With that out of the way, it is fascinating to look at the differences... they are very closely matched in the middle of towns, and on big roads. Once you get out in the countryside, though, small roads can be really far off. In Ahualoa, it's 60m off in some places. In Kamuela, it's a pretty close match except for some 40m offsets in some dryside subdivisions.

I figure that neither dataset is perfect, so it's not necessarily one or the other that's to blame for the mismatch - rather, a combination of their inaccuracies. The USGS data (derived from panchromatic aerial photos) are noticeably divergent in areas like Ahualoa with thick tree cover, which makes sense. The TMKs... well, as they were maintained for decades with ink on linen, they have plenty of reasons why they wouldn't match direct observations.