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Group: Forum Members
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Further to that posted earlier;
GPS altitude displayed is the World Geodetic System 1984 (WGS–84) height above the ellipsoid (geoid height + height above MSL). The GPS altitude is not relative to pressure altitude but is referenced to an earth–centered earth–fixed (ECEF) coordinate system. Pressure altitude is not relative to the same reference frame, but relative to the standard pressure or local pressure settings. Therefore, significant differences can be seen between GPS altitude and pressure altitude.
The system also suffers from at least two factors - vertical dilution of precision (VDOP), and horizontal dilution of precision (HDOP).
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