Fri, 10 Jun 2005 11:28:33 -0700
Dear John, Mark and Al,
Attached, I am sending you a figure of your interest. The table represents the offset between the UTM-PSAD56 and Local Topographic Coordinates (LTP) for the 21 Reference Points that Aerotop installed at the site and to support the road design and to bring a reference-coordinate frame to the AOS.
The table includes for each reference point the North and East UTM coordinate, the altitude of the point with reference to the surface of the elipsoide (ellipsoidal height) and the LTP coordinates after correcting the distances due to the fact the points are at different vertical level over the ellipsoide.
Reference points 1 through 11 go along one of the former candidate roads to connect the OSF and AOS, Reference points 12 to 21 go along the second former alternative road.
Point 0 is located south of San Pedro de Atacama and correspond to a Geodetic-Reference-Point in the Chilean Geodetic Network. This points was used as the master reference for the whole work performed by Aerotop.
As you see at the Chajnantor site (Reference points 10 and 11) show an offset of 5m and 32m in average, with the LTP frame shifted towards the north-east respect to the UTM frame. This offset in the Aerotop frame is due to the fact their rederence-point (point 0) is located at a significantly low altitude in San Pedro. This was done this way because, apparently, the main goal of the selection of the Master-reference-point was to provide a good start point for the two alternative roads to connect the possible locations of the OSF and the AOS.
Therefore, if we are working on the UTM-PSAD56 frame over an area reasonably close to the center of the Chajnantor plateau we can apply a first order offset to the DEM like the averages given above. The second order differences to account for the difference in altitude across the plateau will be in the order of 5 meters (compare points 6 and 21, located at about 4700 m altitude, with points 10 and 11 near the top of the plateau).
All the best,
- 17 Jun 2005