APPENDIX
129
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 1.4 meters 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 Section
6.3.6“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 readings.
For this reason, it is extremely important to maintain the vertical indexing adjustment to
insure proper correction of the horizontal angles.
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