Abstract: A tacheometer includes a distance measuring module whose output indicates the distance to a target by a phase shift in an output signal relative to a reference signal. An angle measuring module has an output that indicates angular displacement by a phase shift in another output signal relative to a different reference signal. A single phase comparison module alternately measures both phase shifts.

Abstract: A tacheometer includes two angle encoders each utilizing a method of eccentricity correction wherein three detectors responsive to encoded angular displacement are arranged linearly along a line passing through the desired axis of angular displacement. Two of the detectors are disposed diametrically opposite each other, at equal distances from and on opposite sides of the desired axis. In the absence of eccentricity the actual axis of angular displacement is the desired axis, and the outputs of the diametrically opposed detectors are complementary. The presence of eccentricity causes the two axes to separate, and produces a corresponding change in the outputs of the diametrically opposed detectors, rendering them non-complementary. The degree of change is measured and used to correct an angular displacement detected by the third detector, located at a different distance from the desired axis.