IIRC the use of GPS to determine heights is a complex topic. You need
to determine the spheroid and geoid separation in relation to the grid
used and so on. Remember that many countires use by the different
versions of these for their mapping and with different origins.

Additionally as identified above there are satellite fix issues.

I suspect then, as a relative method it would be reliable but more
difficult to be reliable as an absolute method.

OC