Awblain:
That's reassuring, since there isn't any "variable gravity" until you're worried about altitudes that reach tens or hundreds of km high.
Clearly the guy is talking about a model that accounts for variation of the gravity field by location rather than using the bog standard 9.81ms⁻². It isn't even slightly controversial. NASA have a satellite mapping this because "the Earth's gravity field is not uniform".
NASA GRACE Mission