Levelling refraction

The classical work on levelling refraction is that of TJ Kukkamäki in 1938–39.

His analysis is based upon the understanding that the measurement beams travel within a boundary layer close to the Earth's surface, which behaves differently from the atmosphere at large.

The Kukkamäki levelling refraction became notorious as the explanation of the "Palmdale Bulge", which geodesists observed in California in the 1970s.

[1][2] Levelling refraction can be eliminated by either of two techniques: An alternative, hi-tech approach is dispersometry using two different wavelengths of light.

Only recently blue lasers have become readily available making this a realistic proposition.