E. Electronic Distance: Combined Constant

1. Combined Constant

An EDM distance is the sum of the measured distance and the instrument and reflector constants, Figure E-1.

Figure E-1
Instrument and Reflector Constants


D = D'+(CI+CP)            Equation E-1  

D: corrected distance
D': measured distance        
CI: instrument offset
CR: reflector offset
k = (CI+CR)

k is the Combined Constant. Because it is internal, most instruments compensate for CI. The surveyor needs only to input the reflector constant; typical values are 0 mm and 30 mm. Over time the TSI may lose calibration and rough reflector handling require determination of a new constant to use.