The Office of the Surveyor General EDM Comparison Facility (Lands Building, Hobart) is designed to verify the capacity of a working EDM (electronic distance measuring device) to deliver measurements that are directly traceable to the National Standard of Length, to an accuracy consistent with the requirements of the
Surveyors Act 2002
Legislative Requirements Pertaining to Legal Traceability of Length for Cadastral Surveying
The Facility provides for a series of measurements, covering the potential range of a test EDM, to be compared directly with measurements of the same lines, made at virtually the same time, with appropriate reference standards.
The testing procedure relies on determination of the difference between a range of directly comparable test and standard measurements - made from the same standpoint - to the same remote prisms, through the same atmospheric envelope at virtually the same time.
The standards used are a steel tape and a Leica TCR803 Total Station. Both are verified Reference Standards of Measurement under the
National Measurement Act 1960
. Measurements are made over six short baselines (6-24m) and six long baselines (200-2863m). The standard distances of the short baselines are determined using the steel tape. The standard distances of the long baselines are determined using the Leica TCR803 Total Station.
In this process, determination of the true length of any of the long baselines is actually irrelevant, as is the relative height or location of either the testing station or the remote prisms. In addition, due to the small variation in time between measurements by the test and certified instruments, the potentially adverse influence of changing atmospheric conditions and structural movement at either end of the baselines is minimised, if not totally eliminated.
The two measurements of any particular baseline are not truly simultaneous. However, repetitive test measurements using the certified reference EDM, at differing times and in widely varying conditions, have demonstrated remarkable consistency in determination of the various baseline lengths, leading to the conclusion that this does not adversely impact on the certification process.
Because the new testing procedure relies on direct comparisons between measurements made by different instruments to a series of fixed remote prisms, it is necessary to establish the magnitude of both instrumental and prism contributions to the so-called 'index correction' for both the test EDM and the standard EDM. For this reason, the test procedure includes the series of short baseline measurements where the test EDM is compared against measurements made with the standard steel tape, rather than the standard EDM, in order to maintain the precision necessary to determine the critical correction factors. Once the individual instrumental and prism contributions to the 'index error' have been established, appropriate corrections can be applied to the test measurements made over the long baselines, to correct for the effect of the 'non-standard' remote prisms.
A Comparison Certificate will be issued if the test confirms that, over all ranges tested, the test EDM satisfies the requirements of the
Surveyors Act 2002
. This Certificate includes a detailed analysis of the test results, including a tabulation of the actual comparisons between measurements made by the test instrument and the relevant Reference Standards. View a
sample comparison certificate
While the first Wednesday of every month is the nominated day for surveyors to submit their EDMs to the Branch for testing, tests may be carried out at other times by special arrangement.
The fees for the EDM Comparison Service were advised in
OSG Circular 1 of 2010
, as follows:
Tests on the first Wednesday of the month:
- single instrument test $100.00 (including GST)
- multiple instrument tests $75.00 each (including GST).
Fees for tests by special arrangement at any other times will be negotiated at the time of booking.
The test EDM must be able to be removed from its own tribrach and be capable of being mounted on a standard 'Leica' type tribrach. It must be submitted together with:
- a prism that can be uniquely identified (serial number or some other permanent identifier)
- a fully charged battery
- an operator's manual.
Please also ensure the following before delivery of the test instrument and prism:
- ppm corrections are set to zero
- instrument is set to read to a prism with a zero prism constant
- instrument is set up to display slope distances
- target locking is disabled
- any sighting vanes are removed from prism (not necessary for mini prisms).
Where the test instrument is a new Trimble, the constants for the instrument's refractive index and carrier wavelength are required to calculate the meteorological corrections. These can be obtained by exporting a job performed using the total station as an .xml file in fixed file format.