There are 2 issues as I understand.
1: The gravitational constant is an assumed standard average value for=20
use at earth's surface for standard results of calculations.
2: The gravitation at earth's surface does vary from location to=20
location for many reasons.

If one is doing hypothetical calculations, and for most practical=20
applications, the constant is fine. For a very few specific applications=20
possibly including the path of a spacecraft and academic problems, the=20
actual value at a specific location may be required.
--=20
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