Surveying
In observations of equal precision, the most probable values of the observed quantities are those that render the sum of the squares of the residual errors a minimum, is the fundamental principle of

D'Alembert's method
Gauss' Mid Latitude formula
Legendre's method
Least square method

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Surveying
The error due to eccentricity of inner and outer axes can be eliminated by

reading both verniers and taking the mean of the two
taking both face observations and taking the mean of the two
taking mean of several readings distributed over different portions of the graduated circle
double sighting

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