Home' Position : Position Aug Spt 2015 Contents to the measured slope distances prior
to reduction of the distances to the
horizontal and the determination of
additive constant and scale factor.
The additive constant is determined
without reference to the published
inter-pillar distances. Therefore it is not
influenced by possible pillar movement.
It should not vary significantly in
subsequent calibrations, provided the
same instrument/reflector combination
is used. The additive constant should be
applied to all measured field distances.
Once set in the instrument, a known
distance should be re-measured to ensure
the sign (positive or negative) of the
constant has been correctly applied or set.
The scale factor generally varies
for subsequent calibrations within
the accuracy specification of the
instrument, because it is dependent on
the instrument's modulation frequency,
which may change with variations in the
ambient temperature. To a lesser extent,
the scale factor can also change as a
result of frequency drift and ageing of the
frequency oscillator. Consequently, if the
scale factor falls within the instrument's
specification, it should not be applied as a
correction to measured field distances.
If the scale factor falls outside the
instrument's specification, the instrument
should be returned to the manufacturer
for service. It is advisable to repeat the
calibration under different climatic
conditions both to confirm the result
and to observe if the scale factor changes
with different ambient temperatures. The
thermometers and barometers used in the
calibration should also be re-calibrated
against a certified standard as an error in
meteorological readings will contribute to
the scale error of measured distances.
This article has briefly outlined LPI's
current EDM baseline infrastructure and
proposed updated best practice guidelines
for EDM calibrations in NSW. These are
anticipated to flow into the next update of
Surveyor General's Direction No. 5.
Note that calibrating an EDM instru-
ment in prism mode does not calibrate
the reflectorless EDM laser. These two
modes generally have different additive
constants and scale factors within any
one instrument, i.e. testing in reflector-
less mode must be performed separately.
Techniques to test EDM instruments in
reflectorless mode are being examined by
research organisations and state authori-
ties. In the interim, ad hoc self-checks will
have to suffice. As such, surveyors should
continue to use professional diligence and
care with such measurements.
Also note that if you measure any
distances that are longer than the longest
line on the EDM calibration baseline, you
should consider the reliability of the ex-
trapolation of your calibration parameters.
When it comes to EDM calibrations, a
millimetre is a long distance. Therefore,
utmost care should be taken to ensure that
the measurements (including temperature
and pressure) are of the highest quality. An
EDM calibration is generally performed
only once a year, so it pays off to do it thor-
oughly --particularly if your measurements
should be questioned by a court of law.
Dr Volker Janssen <Volker.Janssen@
lpi.nsw.gov.au> is with the Survey
Infrastructure and Geodesy branch of
NSW Land and Property Information,
which operates CORSnet-NSW. ■
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in Australia, with over 530 active base stations and more than 1000 regular subscribers.
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