Home' Position : Position Aug Spt 2015 Contents The Surveyor General of New South
Wales is a verifying authority for
reference standards of length
measurements under the National
Measurement Act and responsible for
ensuring that surveyors use verified
To this end, the Surveying and Spatial
Information Regulation requires surveyors
in NSW to verify their Electronic Distance
Measurement (EDM) equipment at least
once a year, using pillared baselines. This
instrument verification establishes trace-
ability of its measurements to the national
standard, and consequently strengthens the
validity of these measurements if ques-
tioned in a court of law.
In order to assist the profession in
meeting this legal requirement, Land and
Property Information (LPI) maintains
several EDM baselines across the state. Lo-
cal organisations (e.g. councils and mines)
often support this function by hosting the
physical baseline infrastructure.
This article briefly outlines the
current status of LPI's EDM baseline
infrastructure and proposes updated best
practice guidelines for EDM calibrations
in NSW. These guidelines are anticipated
to flow into the next update of Surveyor
General's Direction No. 5 (Verification of
Distance Measuring Equipment).
EDM instrument errors
The calibration of an EDM instrument
is performed in order to determine the
instrument errors, which can be used to
monitor its performance and reliability
over time and assess its precision against
the manufacturer's specifications. If
significant, these instrument errors
should be accounted for by applying
corrections to measurements taken
subsequent to the calibration.
If the calibration is performed on
a verified baseline (i.e. a baseline with
a current Regulation 13 certificate)
to a prescribed level of precision, the
EDM instrument is considered to
be standardised. The three distinct
systematic errors that may occur in EDM
instruments are the zero error (or index
error), the scale error, and the cyclic error
(or short periodic error).
The EDM instrument correction is
dependent on many variables, including
distance and atmospheric conditions. It is
made up of at least two terms, i.e. the ad-
ditive constant or index correction (a con-
stant term expressed in mm) and the scale
correction or scale factor (a linear distance-
dependent term expressed in ppm).
Additional terms can be added to de-
scribe the instrument correction in more
detail, e.g. non-linear distance-dependent
terms and cyclic error terms. The instru-
ment correction is valid for a specific
instrument-prism combination only.
infrastructure in NSW
On behalf of the Surveyor General, LPI
currently maintains 15 EDM baselines
consisting of between four and seven
concrete pillars across NSW (see Figure
1.). LPI is in the process of rationalising
and improving this infrastructure by
upgrading existing baselines to include
more pillars and building new 7-pillar
baselines. Additional pillars allow more
distances to be observed, thus increasing
redundancy and providing considerably
more reliable EDM calibration results.
All EDM baselines in NSW (current
and those under construction) follow
the Heerbrugg design (also known
as Schwendener design). This design
features an almost equal distribution
of the distances measured in all
combinations over the baseline length as
well as over the unit length of the EDM
instrument. It permits the detection of
all distance-dependent errors, including
Specially qualified LPI legal metrology
staff verify these baselines on a 2-yearly
basis with precise EDM instrumentation
carrying a current Regulation 13 certifi-
cate issued by the National Measurement
Institute (NMI). The associated mete-
orological equipment is also calibrated
against industry standards. This process
determines the 'true' inter-pillar distances
and establishes traceability, because the
EDM baseline becomes a subsidiary
standard of the International Metre.
Most Australian states and territories
have guidelines on how to perform EDM
calibrations. The following sections
update and expand the current best
practice guidelines available in NSW.
Relevant information can be found on
the LPI website, e.g. at http://www.lpi.
Baseline stability is closely monitored
by LPI to ensure that calibrations can
be performed to the required precision.
If significant pillar movement or pillar
damage is suspected, it should be
reported to LPI via EDMcal@lpi.nsw.gov.
au for immediate action. Nevertheless, the
determination of the additive constant is
immune to pillar movement.
Preparation of equipment
• Book access to the EDM baseline using
the free, online EDM Baseline Booking
System -- this is mandatory for all
baselines in NSW.
• Download the latest EDM baseline
measurement report (detailing the
verified distances, reduced levels and
access details) from the LPI website.
• Check the adjustment of levelling bub-
bles on all tribrachs, reflectors and the
total station, and adjust if necessary.
Levelling of the instrument and reflec-
tors is critical during calibration.
• Verify the thermometer(s) and
barometer(s) against a certified
standard. The collection of accurate
meteorological data is essential for a
reliable EDM calibration.
• Ensure that the EDM battery is fully
charged prior to carrying out the
• Mark all reflectors with a unique
identification number. Use only one
reflector for the EDM calibration
• Download a blank EDM calibration
booking form from the LPI website.
Updated best practice for EDM calibrations in NSW
30 position August/September 2015
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