Surveying
The negative sign is assigned to

Reduction to mean sea level
All listed here
Correction for slope
Correction for horizontal alignment

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

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

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