The University of New South Wales
School of Geomatic Engineering
Temperature Induced Changes in the Lines-of-Sight of Two Topcon DL-101C
by Gary Donovan
Supervisor: A/Prof. J. M. Rüeger
Two Topcon DL-101C precision digital levels (Serial Numbers NJ0100 and
NJ0106) were evaluated to determine the changes in the line-of-sight as
a result of temperature acclimatisation.
This was achieved with a modification of the method used by Sheil in
1997. The line-of-sight changes of two instruments were evaluated simultaneously,
with one being the control instrument and the other acclimatising to ambient
(laboratory) temperature after spending eight hours in a temperature test
cabinet at either +45°C or -10°C. Figure 1 shows the experimental
setup, with the two digital levels mounted on the same heavy-duty tripod
using a special mounting bracket.
Figure 1 - Setup of Test Equipment
The difference between their respective staff readings was assumed to be
solely due to the temperature acclimatisation. The temperature induced
change in the staff readings over two sighting distances was used to separate
the line-of-sight deviation into a vertical shift (Dd)
and an angular component (Dg). These are closely
dependent on the temperature induced change in the height and collimation
errors, respectively. The shift and tilt terms are illustrated below
in Figure 2.
Figure 2 -The Two Components of the Line-of-Sight Deviation
There were 10 separate tests involving both digital levels (Serial numbers
NJ0100 and NJ0106) at both sighting distances (2.0 m and 9.8 m), over both
temperature change cycles (cooldown from 45°C and warmup from -5°C).
The experimental data for each test was compressed into one value summarising
the total change in line-of-sight of the instrument. Using the data from
several tests, the shift and tilt terms (Dd
and Dg) for each instrument were determined,
along with their 95% CI's (Confidence Intervals). These results are summarised
in Table 3:
Table 3 - Summary of Results
Note that the cooling cycle for instrument NJ0106 was determined three
times, but the three results are partly correlated.
The shape of the graphs obtained agree well with other researchers.
The calibration of the line-of-sight deviations cannot be directly compared
with previous results, owing to the substantially different techniques
used. However, they are similar in magnitude and sign to testing by other
researchers (eg. Schauerte (1997)).
The expansion of the digital level's case is probably a significant
component of Dd in the table above. However,
even extreme refraction effects are minimal when compared with the angular
deviation of the line-of-sight.
Several unexplained measurement anomalies were encountered during the
tests. Figure 4 below displays some of these. Each "Test
DL Height" data point represents one measurement in the first 30 minutes
of the test and the mean of two in the remaining 3.5 hours. The precision
of a data point is +/- 0.044 mm in this test with a sighting distance of
10.9 m. The scatter is worst between 30 minutes and 150 minutes.
The graph shows a 0.06 mm downward tooth between 32 minutes and 51 minutes
and another (0.05 mm) between 118 minutes and 150 minutes. A (permanent)
drop of 0.11 mm occurs between 149 minutes and 162 minutes. The figure
also shows that two consecutive measurements (being a mean of two measurements)
can differ by as much as 0.27 mm. The reasons for these anomalies
are not known. The anomalies should be investigated further to establish
their inpact of the use of the DL-101C for precise digital levelling.
Figure 4 - Graph of Some Measurement Irregularities
The testing procedure was significantly aided by the writing of DLay, a
small program to control the DL-101C by serial cable. Both the DL-101C
remote control interface and DLay are very primitive in nature, and there
is great scope for improvement in both.
For more information, contact:
A/Prof. J. M. Rüeger (supervisor)
School of Geomatic Engineering
University of New South Wales
SYDNEY NSW 2052
Mr. G. Donovan (researcher)
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«» revised 16 January 2000
Original: Gary Donovan
«» Last revision: Gary
© 1999 School of Geomatic Engineering, UNSW, Australia.