Calibration of Electronic Distance Measurement Instruments
Western Australian Land Information Authority ABN 86 574 793 858
Calibration of Electronic Distance Measurement Instruments
1. Introduction
An Electronic Distance Measurement (EDM) instrument is calibrated on a baseline to
determine instrument constants and errors.
A series of measurements on a baseline can also be used to check the performance and
reliability of the instrument and to assess its precision against the manufacturer’s claims
and specified minimum standards.
There are a number of sources of error inherent in surveying equipment. This procedure
concentrates on those found in EDM equipment. The three distinct systematic errors, which
may occur in EDM instruments, are:
• zero constant or index error;
• scale error; and
• cyclic or short periodic error
It is beyond the scope of this procedure to identify all sources of error inherent in surveying
equipment. Surveyors should be aware of the limitations of the equipment they use and
ensure that it is well maintained and regularly checked.
2. Controlling Legislation
Matters relating to the accuracy of surveys are detailed in Regulation 5 of the Licensed
Surveyors (Guidance of Surveyors) Regulation 1961 that states:
"The necessity for the greatest practical accuracy cannot be too strongly impressed upon
surveyors. In many instances no means exists by which errors can be immediately
detected. If therefore, it is found that a Surveyor's work is not up to the necessary standard
of accuracy, or is generally unreliable, or that he has in other respects neglected to comply
with these regulations, he will be dealt with under Section 21 of the Act".
3. Regulation 13 Certificate
A verifying authority is empowered to issue certificates under Regulation 13 of the National
Measurement Act that attest to the verification of a standard of measurement. Such a
certificate is evidence of the matters stated in it and may be received as evidence in any
court of law.
An increasing number of measurements are being made for regulatory purposes and these
require traceability within the meaning of Section 10 of the National Measurement Act, for
example, EDMs and Surveyor's tapes.
The Surveyor General of Western Australia is the verifying authority for length in this State.
Surveyors and other clients requiring this service should contact the Registration Services
Branch of Landgate. Western Australian Land Information Authority ABN 86 574 793 858
4. Verification of Electronic Distance Measurement (EDM)
The following errors are determined by a verification:
4.1 Zero constant or index error.
All distances measured by a particular EDM instrument and reflector combination are
subject to a constant error; caused by three factors:
• electrical delays, geometrical detours and eccentricities in the instruments
• differences between the electronic centre and the mechanical centre of the
instrument; and
• differences between the optical and mechanical centres of the reflector.
This error may vary with changes of reflector, or after jolts, or with different instrument
mounting and after service. It is an algebraic constant to be applied directly to every
measurement.
4.2 Scale Error
Scale error is proportional to the length of the line measured and is caused by:
• internal frequency errors, including those caused by external temperature and
instrument "warm up" effects;
• errors of measured temperature, pressure and humidity which affect the velocity
of the propagation; and
• non-homogeneous emissions/reception patterns from the emitting and receiving
diodes (phase inhomogeneities).
4.3 Cyclic Error
The precision of an EDM instrument is dependent on the precision of the internal phase
measurement.
Unwanted interference either through electronic/optical cross talk or multi-path effects of
the transmitted signal onto the received signal causes cyclic error. The major form of the
cyclic error is sinusoidal with a wavelength equal to the unit length of the instrument.
The unit length is the scale on which the EDM instrument measures the distance, and is
derived from the fine measuring frequency. Unit length is equal to one half of the
modulation wavelength (Rueger 1984). The magnitude of the cyclic error can be of the
order of 5 - 10 mm, but it will vary depending on the actual length measured.
5. Western Australian EDM Baselines
5.1 Curtin University Baseline
In 1995 the Surveyor General reconditioned a 10 pillar baseline at Curtin University which
had been in use for approximately ten years. Renovation included the placement of two
additional collinear pillars making the baseline a twelve pillar base. New stainless steel
pillar plate tops have been provided to all pillars, with a 5/8" whitworth threaded bolt at the
centre of each pillar top. In 2005 a replacement Pillar 11A was established following the
destruction of Pillar 11. Western Australian Land Information Authority ABN 86 574 793 858
This baseline is situated on Curtin University property parallel to Kent Street, Bentley. The
baseline is managed by Landgate.
The baseline is accessible by any vehicle at all times via the track alongside the length of
the baseline. Permission is not necessary to use the baseline. Any conflict of usage should
be decided on a first to occupy basis.
A software program [BaselineWA Version 2009.10.26.0] has been developed specifically
for the verification of EDM instruments over the baseline.
5.2 Kalgoorlie Baseline
This EDM calibration baseline is situated on the Department of Mines and Petroleum,
Explosives Reserve in Piccadilly Street, Kalgoorlie.
Access to the facility is available during normal business hours after obtaining the required
entry permission. Any conflict of usage should be decided on first to apply basis.
Contact Reserve Manager, on 9091 7590 to make appointments to go onto the reserve.
The following conditions will need to be met for continued access to the site.
• Requests for access, are made preferably at least one week in advance.
• The number of people coming onto the reserve to conduct calibration work is kept to
an absolute minimum.
• People conducting the calibration must be accompanied at all times by a reserve
officer while on the reserve.
The baseline consists of eight pillars set out such that all pillars are collinear.
The baseline is readily accessible to 2WD vehicles when dry.
A software program [BaselineWA Version 2009.10.26.0] has been developed for the
verification of EDM instruments over the baseline.
5.2 Busselton Baseline
This baseline was completed in 2009 and is situated on Reserve 44755. It runs parallel to
and to the North side of the Busselton Bypass Road, 1.3 Kilometres west of Redgum Way.
The baseline is managed by Landgate.
The baseline is accessible by any vehicle at all times via the track alongside the length of
the baseline, with entry via the west end of the truck bay. Permission is not necessary to
use the baseline. Any conflict of usage should be decided on a first to occupy basis.
The baseline consists of six concrete pillars set out such that all pillars are collinear. Western Australian Land Information Authority ABN 86 574 793 858
A software program [BaselineWA Version 2009.10.26.0] has been developed specifically
for the verification of EDM instruments over the baseline.
6. Minimum Standard for the Uncertainty of Calibration
The National Standards Commission now incorporated into the National Measurement
Institute, states that the minimum standard for the uncertainty of calibrations of an EDM
instrument used for cadastral surveys should be:
± (4.0mm + 20ppm) (5.1)
at the 95% confidence level. This recommendation means that an instrument correction is
derived for a distance meter/reflector combination from measurements on a certified EDM
baseline and that the uncertainty (against the National Standard) of this instrument
correction (IC) shall not exceed ± (4.0mm + 20ppm). Subsequent to a calibration, the
derived instrument correction is applied to distance measurements, thus bringing the
distance meter readings in line with the National Standard of length. In terms of standard
deviation, the instrument correction must be accurate to at least ± (2.0mm + 12ppm) (5.2)
It is important to realise that the uncertainty and standard deviations listed above refer to
the accuracy of the instrument correction and not to the precision of a distance
measurement. The expression at (5.2) does not relate in any way with the accuracy
specification quoted by manufacturers for their instruments.
7. Verification Method
The verification method involves the measurement of a set of segments on the EDM base
to determine the existence and magnitude of any errors present. This level of verification
will typically be undertaken when the equipment is to be used for precision survey work
requiring an accuracy of distance measurement of greater than 1 part in 12 000.
8. Verification Procedures
8.1 Booking Sheets
• Curtin University Baseline
• Kalgoorlie Baseline
• Busselton Baseline
8.2 Measurement Procedure on Baselines
Ascertain the unit length of the instrument (half of the modulation wavelength of the fine
measurement) and record on the booking sheet. This length should be provided under the
technical specifications for the instrument in the manufacturer's handbook.
8.3 Sequence of measurements for a complete verification.
• CURTIN UNIVERSITY BASELINE
Place EDM on pillar 2 and measure to pillars 5, 6, 7, 8, 9, 10, 11A and 12 in turn. Then
shift EDM to pillar 3 and measure to pillars 12, 11A, 10, 9, 8, 7, 6 and 5 in turn. This
sequence requires the reflector to be moved up and down the line only once. Western Australian Land Information Authority ABN 86 574 793 858
• KALGOORLIE BASELINE
Place EDM on pillar 1 and measure to pillars 3, 4, 5, 6, 7 and 8 in turn. Then shift EDM
to pillar 2 and measure to pillars 8, 7, 6, 5, and 4 in turn. This sequence requires the
reflector to be moved up and down the line only once.
• BUSSELTON BASELINE
Place EDM on pillar 1 and measure to pillars 2, 3, 4, 5 and 6 in turn. Then shift EDM to
pillar 2 and measure to pillars 6, 5, 4, 3 and 1 in turn. This sequence requires the
reflector to be moved up and down the line only once.
If required, higher order calibrations can be obtained for EDMs. These require several
hours to complete the observations. Please contact the Manager of Geodetic Survey,
Landgate for more information.
8.4 EDM instruments, thermometer and barometer are to be shaded by an umbrella in
sunny and rainy conditions. Only one thermometer and barometer will be needed. A
psychrometer or hygrometer may be used to measure humidity for a more accurate
determination of atmospheric correction.
8.5 Level EDM instrument and reflector carefully on all stations. Start with all foot screws in
the mid position. The stainless steel pillar tops have been set level on pillars.
8.6 Measure all heights to the instrument and prism from the pillar plate accurately to +/-
1mm.
8.7 If possible the instrument should remain switched on during the whole calibration
(instruments with own thermostatically controlled oscillators must remain switched on
during the whole calibration).
8.8 The same reflector, reflector mounting and tribrach should be used for all
measurements. The reflector must have a unique identification (serial number) which must
be entered on the booking sheet.
8.9 All calibration measurements can be taken in either daytime or nighttime but a mixture
of day and night measurements is not acceptable.
8.10 Meteorological field equipment will have current calibration status.
8.11 On each line, four separate distance measurements should be taken as a minimum,
with repointing after each measurement. Pointing can be optically or electronically
performed as prescribed by the manufacturers.
8.12 Atmospheric Correction Control - Atmospheric corrections using corrected observed
temperature and pressure should preferably be entered into the EDM instrument. This is
essential for Pulsed EDM instruments. Temperatures and pressure should be noted for
every distance measured. Observers MUST note on the booking sheet whether the Western Australian Land Information Authority ABN 86 574 793 858
atmospheric correction control is set to zero, or has been set according to the prevailing
temperature and pressure.
9. Reduction and Interpretation
An EDM Calibration software program [BaselineWA Version 2009.10.26.0] has been
developed by Landgate for the calibration of EDM instruments against standard baselines.
The calibration of EDM instruments is carried out to determine the instrument corrections to
be applied to measurements and to ensure its reliability. On-line help provides the user
with the instructions necessary to run this software application. The software can be freely
downloaded from the Landgate Website.
The calibration results and measurements for each EDM instrument and baseline are
stored in a database for future reference and legal traceability. The CurtinA, Kalgoorlie and
Busselton baseline details have already been included in the database. A selection of EDM
instruments and reflector makes and models are also included.
The instrument details, measured distances and observed meteorological details are
entered interactively by the operator. After the observed data is reduced to obtain
horizontal distances and their associated variances, a least square adjustment is
performed. The adjustment is made as suggested by Dr J.M. Rueger (Rueger 1984) for
modeling systematic errors in EDM measurements.
The program produces several reports for analysing an EDM instrument calibration and for
analyzing a calibration of a baseline. A certificate which summarises the results of an EDM
instrument calibration is also produced.
To obtain in Western Australia a Regulation 13 Certificate for the purposes of legal
traceability to the Commonwealth standard of length, the instrument must be submitted to
Geodetic Survey at Landgate for verification and certification.
The minimum standards for the uncertainty of calibration are described in terms of
Recommendation No.8 of the working party of the National Standards Commission (now
NMI) on the calibration of EDM Equipment of 1 February 1983. All uncertainties are
specified at the 95% confidence level.
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