Summary
Just as the gravity of the moon causes tides, these variations in gravity have a profound affect on sea level and create stationary crests and troughs up to 500 feet. Since our traditional methods of surveying measure altitude as a distance above sea level, you can see where this could be problematic - with variations up to 500 feet, you can't guarantee a common starting point. At higher altitudes, above the terrain, relying on GPS altitude is even more dangerous... because all the other air traffic is relying on barometric altitude in accordance with FAA and ICAO rules.
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Extract
The Dangers of Gps Altitude
As aviators, we rarely think about the accuracy of our instruments. When you look down at your airspeed indicator and see 200 KIAS1 unless you have a specific reason to doubt the equipment (icing, etc.), you can reasonably assume that the aircraft is moving at 200 KIAS. When your VVI tells you that you're descending at 300 feet per minute, tha...
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