Earth has an ovel shape. Then how sea level at meridian and on poles can be equal?

Question

Everywhere, we are quoting height from MSL. If an aircraft is flying at a height of say, 10Km above sea level. Then will it be equal height the core (centre) of earth - if it is flying over meridian sea or a sea near Polar region ?

Answer 1

maybe, but probably not (see below)

1) earth is an oblate spheroid
2) sea level at the meridians and at the poles is different, just as the sea level is different on the east and west coasts
3) MSL = Mean Sea Level, where "mean" means "average"
see http://en.wikipedia.org/wiki/Sea_level

pilots also use ASL/AGL measurements (above sea level, above ground level)

the center of the earth's mass (also its center of gravity) is not the same as its geometric center, and neither of those is its magnetic center, and the core of the earth isn't perfectly spherical -- so "equal height with respect to the core" doesn't really express anything meaningful.

as all these things are in motion even with respect to each other, it is conceivable that an altitude of 10km ASL at the meridian might at some instant also coincide with the distance from the core at the pole -- IF the MSL altitude that corresponded to the ASL altitude being flown at the meridian is maintained across the pole.

otherwise, ASL would become AGL, and the aircraft would be significantly higher in the atmosphere at the pole than it had been at the meridian, virtually guaranteeing that confluence of the distances from the core was impossible

Answer 2

in an aircraft sea level is based on atmospheric pressure and adjusted according to account for the variations. height above the ground or sea can be determined by using the medium of sea level at the current pressure.