I've found significant altitude errors using a GPS and the following quotes found on the internet will explain why. From my experience of hiking in the mountains of New Hampshire an aneroid altimeter will vary with atmospheric pressure about 200 feet for a change of 0.2" of mercury so you have to continually set it at known waypoints, just like setting a frequency standard against a known reference, and then it will be 'accurate' for some length of time, and then you set it again. GPS altitude will be off but it should be fairly consistent in spite of the changing atmospheric pressure. The earth neither spins at a constant rate nor is it a perfect sphere. Maybe we need to trade it in for a newer model. ;-)
"GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. First, the geodetic model of the earth can have much more than this amount of error at any specific point and second, you have the GPS error itself to add in. As a result of this combined error, I am not surprised to be at the seashore and see -40 meter errors in some spots." "We have to make some assumptions about the shape of the earth. WGS84 has defined that shape to be an ellipsoid, with a major and minor axis. The particular dimensions chosen are only an approximation to the real shape. Ideally, such an ellipsoid would correspond precisely to "sealevel" everywhere in the world. As it turns out, there are very few places where the WGS84 ellipsoid definition coincides with sealevel. On average, the discrepancy is zero, but that doesn't help much when you're standing at the water's edge of an ocean beach and your GPS is reading -100ft below sealevel. The deviation can be as large as 300ft in some isolated locations. When the National Marine Electronics Association came up with the NMEA standard, they decreed that altitudes reported via NMEA protocol, shall be relative to mean (average) sea level. This posed a problem for GPS manufacturers. How to report altitudes relative to mean sea level, when they were only calculating altitude relative to the WGS84 ellipsoid. Ignoring the discrepancy wasn't likely to make GPS users very happy. As it happens, there is actually a model of the difference between the WGS84 ellipsoid and mean sea level. This involves harmonic expansions at the 360th order. It's a very good model, but rather unusable in a handheld device. It was determined that this model could be made into a fairly simple lookup table included in the GPS receiver. The table is usually fairly coarse lat/lon wise, but the ellipsoid to mean sea level variation, known as geoidal separation, varies slowly as you move in lat/lon." -Arthur _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
