Just been watching this interesting video of how to calculate the height things are above sea level taking into the earth not being round and the dense spots, and the effect of gravity has on mass.
[video=youtube;q65O3qA0-n4]http://www.youtube.com/watch?v=q65O3qA0-n4#t=182[/video]
[video=youtube;q65O3qA0-n4]http://www.youtube.com/watch?v=q65O3qA0-n4#t=182[/video]