It is clear from the table that dual axis compensation has the most benefit when the elevation
of the target is greater then 30° and the axis is tilted more than 10". The entries indicated in
bold in the table show, in fact, that for many common surveying applications i.e. target
elevation <30° and axis error <10", virtually no correction would be required. Dual axis
compensation is especially suited then for applications where the sights are very steep.
Even though the compensators can correct horizontal angles for vertical axis errors,
it is still important to use care in setting up the instrument.
Centering error, for instance, cannot be corrected by the compensators. If the vertical axis is
tilted by 1’ with the instrument 1.4meters above the ground, a centering error of approx. 0.4mm
will result. The maximum effect of this error at 10m is about 8" of horizontal angle error.
In order to maintain the increased accuracy possible through dual axis compensation, it is
necessary to keep the compensators in proper adjustment.The compensators must agree with
the actual level condition of the instrument. Through various environmental stresses, the
agreement between the level condition sensed by the compensators and the true level condition
of the instrument may be disturbed. In order to reestablish the correct relationship between the
compensator and the true level condition of the instrument, it is necessary to carry out the
vertical indexing procedure listed on chapter 14.3.6 "Adjustment of vertical Angle 0 Datum".
This adjustment will both reset the vertical index (cause a direct + indirect zenith reading to the
same elevation to equal 360°) and zero the level reference for the horizontal compensator.
While correct vertical angles can be obtained by averaging direct and indirect reading even
when the index is improperly adjusted, the same is not true for horizontal angles. Since the
vertical axis error is fixed for a given setup, its effect cannot be removed by averaging two
For this reason, it is extremely important to maintain the vertical indexing
adjustment to insure proper correction of the horizontal angles.