A precisely measured base line is required to find the instrument constant of an EDM. In determining the instrument constant, repeated measurements (say 10 to 15) of the base line are be taken using the EDM. Meteorological data during measurement should be gathered with extreme care. The average of the EDM measurements corrected for meteorological conditions and for slope of the line should agree with the length of the base line. The difference between the two measurements represents the instrument offset.

If a reliable baseline is not available and if it is not feasible to measure a line, instrument offset is calculated by measuring a line of unknown length in several sections. Let the distance D be divided into arbitrary increments d₁ , d₂ , ….., d_n (having atleast n =2) (Figure 10.5). Measure the total length of the line using EDM and then measure each segment separately. All distances are to be corrected for meteorological conditions and slope. Let the instrument offset be c₁ , then the total distance is D+ c₁ and the increments are d ₁ + c ₁ , d₂ + c ₁ , ….., d_n + c ₁ . Thus,

D+ c₁ = ( d ₁ + c₁ )+( d₂ + c ₁ )+ …..+( d_n + c ₁)