"Jim Macklin" writes:

If I take-off in a airplane with no radio and fly through a
cold front in VFR, the altimeter setting is likely to change
enough to have a 500 foot or greater error with a 100 mile
flight. A GPS can be used to get closer than that.

Unfortunately, no, it cannot.

Probably, most NORDO flights are at or below 3,000 AGL under
VFR. Terrain clearance is more critical to such a flight
based on the altimeter than is the mid-air collision problem
since the "rule" only applies above 3,000 AGL.

GPS altitudes are especially dangerous to use near the ground, becuase
they are so inaccurate.

Someday, the rules may change, but within today's rules,
using GPS "as the best available" or only source is better
than no source at all.

True, if you are far from the ground and you cannot see the ground.
Don't trust it to keep you clear of terrain at low altitudes.

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