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Cadastral reform in Malaysia: A vision to the 2000s
GPS-based cadastral survey
Nowadays, the precision GPS receivers can provide coordinates which are sufficiently accurate for cadastral purposes in rural areas. More importantly, these receivers offer an opportunity to significantly lower the cost and time typically required for cadastral surveys. But, the most fundamental aspects that have to be considered in this study is 'the GPS practice for the use of GPS in cadastral surveying so that is similar to those that already exist for EDM/total station procedures. This will provide means of ensuring the highest quality practices are adhered to which surveys pertaining to land boundaries and title. Therefore, in designing and testing a GPS methodology for cadastral surveying, the following criteria should be adopted, Barne, et.al. (1996).
- speed (must significantly outperform current approaches)
- cost (must significantly reduce current unit survey costs)
- appropriate (must be within the reach of local surveyors)
- realistic accuracy (match real needs )
- simple field operation (data collection must be simple to allow for variable
field conditions).
Thus, the GPS cadastral survey will includes:
- The selection of GPS hardware./software
- The testing/calibration of the GPS equipment
- GPS control survey procedures
- The manners in which the GPS results are processed within a least squares
procedure so that the coordinates can be 'derived' without ambiguity.
- GPS Calibration Tests
The GPS system testing/calibration is considered as a prerequisite for proving a competence so that the GPS derived coordinates are of uniformly high quality, i.e. it has 'legal traceability'. In order to fulfill such requirements, a GPS calibration network, consisting of few points, should be established to enable government and private surveyors to calibrate their GPS units (receivers, antennas, firmware and software) by providing a set of accessible points with accurately known coordinates. Thus, it provides the basis for a standard test that can be applied to all GPS receivers. This kind of test will become increasingly important as private surveyors embrace GPS technology and the GPS market expands. In this study, a series of GPS baseline test have been carried out using three (3) dual frequency Leica 300 System at the existing EDM baseline calibaration test site in UTM campus. The EDM test site comprises of 6 pillars separated at specified intervals with the shortest and longest distance of about 10 meters and 900 meters , respectively. The baselines have themselves been calibrated against a standard, and hence can fulfill the requirements of legal traceability of GPS-derived distances. In the calibration test, we have found that the differences between GPS and 'EDM published true values' for pairs of the receivers of less than 10 mm which indicates that the GPS equipment set being used are in good condition.
- GPS Network Test
The GPS network test should also be performed to assure the operation of the GPS instrumentation for the purpose of determining high accuracy relative coordinates. Therefore, prior to any cadastral fieldwork with a particular GPS unit, it must be calibrated against the calibration network. The GPS network test can be carried out on an annual basis or when processing software (new version) and/or firmware (new version) is changed. The test is also the most realistic form of test as it ensures that the results for all inter-antenna distances can be checked. It is recommended that the network should include a minimum of 3 existing 1st. order GPS stations (as published by DSMM) and all stations have sky visibility of at least 90%. The relative accuracy of better than a + bL (in mm) should be adopted in the GPS network test; where a= 5mm, b = 2ppm and L = baseline length in kilometres. In this study, three (3) GPS stations (known coordinates) namely GP12, GP13 and M311 separated about 30 km away, have been used as GPS test network using the dual frequency GPS receivers . All points have been occupied for more than one hour observation span to track a minimum of five GPS satellite with cut off angle 15 degrees. The baseline processing and minimal constraint adjustment is carried out using SKI software in WGS84 system. Subsequently, the coordinate transformation, (i.e from WGS84 => MRT=>RSO => CS) is performed using the procedures outlined in previous section. The results have shown that the difference between the adjusted coordinates and the published coordinates is quite small (about 5 cm). Also, the newly GPS derived distances of two baseline, i.e. GP13-GP12 and GP13 - M331 is within the allowable limits (less than 70mm per 30 km) which also indicates that the GPS equipment set being used are in good condition.
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