# Why not measure elevation from the center of the earth?

In trying to understand Geoid vs Ellipsoid height, I ran across this page.

One of the claims is:

Zero elevation as defined by Spain is not the same zero elevation defined by Canada

Basically, they are indicating that MSL is not constant and that makes comparing things harder.

So my question is: why not measure elevation as a radius from the center of the earth?

I guess I'm proposing `{lat, lon, radius}` to define a point on or near the earth. This could eliminate many problems in my thinking. (Let's use inertial center of the earth as our reference point...)

Who cares that the topography of the earth is not constant? It wasn't constant in the first place, and to get height above ground, you just need some terrain data and a simple subtraction. I guess I'm proposing using one common reference point that doesn't change.

• How much have you read about datums? And more specifically ECI and ECEF? Where is the center of the earth that is a fixed point that doesn't change? – Chris W May 22 '14 at 20:23
• If you don't use some sort of gravity-related origin, water can flow uphill! – mkennedy May 22 '14 at 20:40
• @ChrisW Nothing. I'll read your links. When I said "doesn't change" I meant with respect to the surface of the earth. Obviously, that is not entirely true, as earthquakes or tectonic plate movement and the like can change things, but I was thinking it would be FAR more constant than the average level of the ocean for the last 19 years. I am anticipating something similar to ECEF, but using lat/lon to allow folks to intuitively know the location in question. ECEF is hard to know without converting. – kmort May 22 '14 at 20:41
• Since we can't physically observe the center of the earth, we have to use other kinds of observations, mathematics, and models (collectively known as geodesy) to approximate the shape of the earth, with varying degrees of accuracy (and no one model is perfect for every place on earth, that's why there are so many). – blah238 May 22 '14 at 20:43