CompuServe Messages

,<<I wonder how they're getting that kind of accuracy with GPS? Maybe they have access to the military version? Or are they just interpolating from known benchmarks.>> In static differential mode, with 12 channel receivers, relative height difference between 2 points should be computable to around 0.5 mm plus 1ppm of the baseline length at the 2drms confidence level. No "military version" required. I've been getting closures with conventional control to better than 1cm with a few seconds observation on each site using "on the fly" techniques which are much cruder than the static obs proposed. You do have to remember, though, that GPS heights are relative to the ellipsoid, and not to the geoid, so the observers will need a *very* good model when reducing the obs. BTW: are you the Alan Balmer who used to work for Geco? Rob