From Mechanics

In part a) the weight of the man at the top of the mountain is found to be 785 N.

At sea level \displaystyle g=9\textrm{.}81\text{ ms}^{\text{-2}}, and his weight is \displaystyle mg giving \displaystyle 9\textrm{.}81\times 80=785\text{N}

The difference is negligible and variations due to the fact that the earth is not perfectly round (spherical) create a larger error than the above approximate calculations.