CompuServe Messages

Advances sure have been made, Don. Gravitational models are available now as series up to order 180, with over 32,000 coefficients. You can even model in time terms, these days, to take care of tidal distortions in the crust. We can easily do relative positioning to a couple of millimetres plus 1-2 parts per million of the baseline distance up to continental distances. With the right GPS receiver and SA switched off or compensated for ( eg, military receivers), we can do single point positioning withing the define WGS 84 reference frame to a couple of metres, no problem. The nice thing about GPS, or any satellite system, is that the ellipsoid is not used in the position computation. Positions are computed in terms of a gravitationally geocentric cartesian coordinate system, and only trandsormed to lat/long or grids for those who still need to work in terms of these coordinates. I retriangulated part of Northern Brazil in 1986 using GPS. The original trig had been put in using Hiran and Transit Doppler. We found a 56 m bust in the original net, so I would agree with your comment about the '60's <g>. Rob