Levelling refraction refers to the systematic refraction effect distorting the results of line levelling over the Earth's surface.
In line levelling, short segments of a line are levelled by taking readings through a level from two staffs, one fore and one behind. By chaining together the height differences of these segments, one can compute the total height difference between the end points of a line.
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. When measuring over a tilted surface, the systematic effect accumulates.
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.