Questions and answers about cross hairs and EDM measurements.

A: Precise angles should be measured with a target provided with the prism by the manufacturer. Such targets are designed to coincide with the prism’s plumbing point, which may or may not coincide with the apparent center of the prism. For precise work, be aware that there may be an offset on the order of tens of millimeters (40 mm is not unusual) between the location of the plumbing point and the apparent intersection of the three plane surfaces at the back of the prism glass that come together in the “star shape.” Note that the physical location of the point of the back of the prism versus the plumbing point is not the alignment discrepancy you are dealing with; rather it is the apparent center versus the plumbing point.