CompuServe Messages

High Bill: I've been away for a while, and probably missed a lot on this forum. To answer your question about differential corrections: yes, the corrections would still be valid at the accuracies you're talking about. The height difference would probably introduce more errors than the 50 km horizontal difference. Dynamic offshore surveys achieve 5 m or better with ranges to differential stations of well over 500 km. Some suppliers claim no degradation out to 1000km. What you do need to be sure of is that the diff stations you're using are properly installed and monitored. I've seen instances of multipath effects of up to 15-20 metres corrupting the diff corrections broadcast from the reference stations. If you're moving, you've also got to watch out for the update rate of the corrections and the time lag from signal reception at the reference antenna until the corrections update your position computation. Within limits, selection of the nearest correction transmitter doesn't matter. Some systems autoselect the nearest. Some allow manual selection, and some (the more expensive) take a weighted mean of all reference stations " in view". Trimble has been working on a Wide Area DGPS (WADGPS) system which would cover a major continental mass with about 4-5 reference stations. It might interest you to know that I've just completed a land survey using both conventional techniques and a Trimble SSE Total Station and Rover pack. The conventional survey took 4 days and closed out to 5 cm ( 2 ins) at the 2 sigma level. The GPS work was done using "stop /go techniques. It took 1 day. The biggest bust with the conventional control was 38 mm ( yes, millimetres) in Northings. This was out of 200 comparison points, so it wasn't a fluke. For your info, I've just applied to CIS, on behalf of the Hydrographic Society, to run a "Survey and Hydrography" Forum. If the proposal is approved, we intend to devote an entire section to GPS, covering everything from high precision geodesy to casual leisure use. I'll keep you posted as to whether it's going ahead. Rob