Hi Dave, On Sat, 2008-08-30 at 11:09 +0100, David Ackrill wrote: > Michael Baker wrote: > > > However, after completing its self-survey it thinks > > its elevation is 11.2 meters when the actual elevation > > of its antenna (on my house roof) is 28.4 meters. > > I've not found any GPS receiver, available to the general pubic anyway, > that correctly reports its height above sea level. So, I tend to ignore > that reading, personally.
Due to geometric reasons the GPS altitude/height-accuracy is worse by a factor ca 1.8, than the horizontal (2d) accuracy. This gives the basic reason the vertical is worse. Other factors contributing - the earth is flat (!?) ie heights from maps of sea levels etc can be very accurate. - user confusion of ellipsoid height (that GPS works in) and geoid height (mean sea level height). You need to know/check which height is given by the GPS in each "message" you are looking. - inaccurate geoid model in the GPS to translate ellipsoid height to Geoid height. - modern high sensitivity receivers will happily track signals that have bounced a couple times between floor and ceiling, which of cause gives even worse height accuracy. > Dave (G0DJA) -- Björn _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.