17.1 Checking and adjusting of instrument constant
Normally, the instrument constant does not have discrepancy. It is recommended you measure and
compare with an accurately measured distance at a location where the precision is specifically
monitored on a consistent basis. If such a location is not available, establish your own base line over
20m (when purchasing the instrument) and compare with the data measured with newly purchased
In both cases note that the setup displacement of the instrument position over the point, the prism,
baseline precision, poor collimation, atmospheric correction, and correction for refraction and earth
curvature determine the inspection precision. Please keep in mind these points.
Also, when providing a base line in a building, please note that the difference in temperature greatly
changes the length measured in the building.
If the difference between the measured length of the baseline and the actual length exceeds the range
of the nominal accuracy, change the instrument constant of the prism mode according to the following
1) On a basically horizontal line of about 100 meters, line AB, establish point C. Measure each
distance, AB, AC and BC, about 10 times, and calculate the average value for each.
2) Repeat Step 1 several times. Then calculate the variance with the current instrument constant (ΔK).
ΔK = AB - (AC + BC)
3) Calculate the new instrument constant according to the formula below. Then reset the instrument
constant according to Section 17.4 “How to Set the Instrument Constant Value”.
New instrument constant = current instrument constant + ΔK
4) Once again, measure the baseline and compare the results to the baseline’s actual length.
Repeat the same procedure in non-prism mode as well.
5) If the results of the measurement conducted in Step 4 exceed the range of the nominal accuracy,
contact TOPCON or your TOPCON dealer.
Note: Each of the Prism mode and Non-prism mode has instrument constant.
Check and adjust in both non-prism and prism modes to obtain an instrument constant for
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