What is the earths radius at 70 degrees north?

Question

can you also show how to do it?

I actually need it for 16 degrees north now.

Answer 1

I assume you don't want the spherical-earth model since that's the same for any latitude. For a better but not perfect approximation to the true earth, the ellipsoidal model should do. Unfortunately this gives you at least 3 radii to choose from, and there's even some, er, latitude in how you define latitude. See the ref. Basically you can approximate the distance-from-the-center radius as
a*sqrt[(1-e^2)^2 sin^2φ + cos^2φ] / sqrt[1 - e^2 sin^2φ]
where φ is latitude, eccentricity e = sqrt[1-(b^2/a^2)], a is the semimajor axis or equatorial radius and b is the semiminor axis or polar radius. Commonly used values for a and b are 6378 km and 6357 km respectively.