Editing Earth radius (section)

===Radius and local conditions===
[[File:Abu Reyhan Biruni-Earth Circumference.svg|thumb|[[Al-Biruni#Geodesy and geography|Al-Biruni]]'s (973 – {{c.|1050}}) method for calculation of the Earth's radius simplified measuring the circumference compared to taking measurements from two locations distant from each other.]]
Given local and transient influences on surface height, the values defined below are based on a "general purpose" model, refined as globally precisely as possible within {{convert|5|m|ft|abbr=on}} of reference ellipsoid height, and to within {{convert|100|m|ft|abbr=on}} of mean sea level (neglecting geoid height).

Additionally, the radius can be estimated from the curvature of the Earth at a point. Like a [[torus]], the curvature at a point will be greatest (tightest) in one direction (north–south on Earth) and smallest (flattest) perpendicularly (east–west). The corresponding [[Radius of curvature (applications)|radius of curvature]] depends on the location and direction of measurement from that point. A consequence is that a distance to the [[horizon|true horizon]] at the equator is slightly shorter in the north–south direction than in the east–west direction.

In summary, local variations in terrain prevent defining a single "precise" radius. One can only adopt an idealized model. Since the estimate by [[Eratosthenes]], many models have been created. Historically, these models were based on regional topography, giving the best [[Figure of the Earth#Historical Earth ellipsoids|reference ellipsoid]] for the area under survey. As satellite [[remote sensing]] and especially the [[Global Positioning System]] gained importance, true global models were developed which, while not as accurate for regional work, best approximate the Earth as a whole.