Guidelines for GPS Cadastral Surveys in Malaysia

Chris Rizos, in partnership with Prof. Majid Kadir (Department of Geomatics, Faculty of Geoinformation Science & Engineering, University of Technology Malaysia, Johor Bahru), have completed a consultancy for the Dept. of Survey & Mapping Malaysia, on the development of "Guidelines for the Use of GPS in Cadastral Survey in Malaysia". The consultancy commenced in late 1997, and concluded with the presentation of a report, and a seminar / "hands-on" workshop at the DSMM on 10-12 May 1999. The resultant guidelines include testing procedures for GPS equipment, and "recommended practices" for field and office procedures.

Publications:

Abstract: The Department of Surveying and Mapping Malaysia (DSMM) in 1997 initiated an investigation into the potential use of GPS for cadastral surveys as part of wider feasibility study of several issues impacting on the introduction of a Coordinated Cadastral System (CCS) for Malaysia. The investigation was restricted to the issue of legal traceability of GPS cadastral surveys, and the associated matters of GPS equipment testing and recommended survey practices for GPS cadastral surveys in Malaysia. As is the case with many countries, the practice of cadastral surveying is regulated by legislation and one of the requirements is that distance measurements are traceable to primary standards of measurement by means of calibration. However, the implications for GPS cadastral surveying are unclear as GPS position is not a defined physical quantity in the normal sense. In Malaysia an alternative solution to the conventional approach of calibration against a standard has been proposed. It has been identified that a set of recommended practices or guidelines regarding the use of GPS for cadastral surveys needed to be developed. In 1999, a set of GPS cadastral survey guidelines for Peninsular Malaysia were recommended to the DSMM for promulgation as 'directions to surveyors'. The guidelines deal with such matters as appropriate survey planning and design, the testing/calibration of GPS equipment, the selection of the appropriate GPS field technique, and the related field and office operation procedures linking the survey to the national control network.

Background: The new Geodetic GPS Network for Peninsular Malaysia was first observed in early 1992 and completed in 1993, and provide a consistent set of coordinates in the 'WGS84 datum'. The network consists of 238 GPS stations with an average spacing of about 30km. Since the early 1990s, GPS surveying techniques have been used in Malaysia for a variety of applications, particularly in providing control for large engineering projects. However, in the last few years, increasing interest has been shown in using GPS for cadastral surveys, particularly in relation to the future implementation of Coordinated Cadastral System (CCS) in Malaysia. However, because carrier phase-based positioning is the most complex of the GPS techniques it is necessary for practitioners to be well trained and educated in the use of such techniques. Furthermore, the special requirements (in terms of legal obligations, etc.) for cadastral surveying means that particular attention must be paid to testing and 'certification' of the technology, and the field/office procedures associated with it. Although such guidelines have been developed in several countries, they have mostly been concerned with control surveying practices. (For example, the Australian Inter-governmental Advisory Committee on Surveying & Mapping's two publications "Standards and Practices for Control Surveys" and "Best Practice Guidelines on the Use of GPS for Surveying Applications, both available from http://www.auslig.gov.au/geodesy/gps/gps.htm.) The DSMM requested that similar Guidelines be developed for Malaysia, but that they be specifically for GPS cadastral surveys and that they incorporate recommended practices which could satisfy, in a pragmatic way, the requirement for "legal traceability". In effect, the objectives of the study were:
(1) Determine whether a pragmatic definition of "legal traceability" could be incorporated in a set of conditions that must be fulfilled by the cadastral survey and through some form of instrument testing.
(2) Determine the utility of a range of possible "tests" that could be regarded as somehow accrediting, certifying, calibrating, validating or verifying GPS systems and their performance, including: (a) using the DSMM's national GPS network, (b) EDM baselines, (c) so-called "zero baseline" tests, and (d) special quality assurance practices during the GPS survey.
(3) A set of "standards & specifications" (or "recommended practices" or "guidelines") for cadastral surveys relating to: o Planning a GPS survey and how the coordinates are to be determined through appropriate connection to survey control: (a) Testing/calibration of GPS equipment (see above), (b) Field procedures for operating the equipment, documentation, quality assurance and verification procedures, etc., and (c) Office procedures for data reduction and result submission.
(4) Recommend the appropriate implementation strategy, including the development of training materials.

GPS Equipment Testing For Cadastral Surveys: In general, measurements are only "legal" if they are "traceable" to primary standards of measurement. Accordingly, the definition of "legal traceability" adopted by the DSMM is that a GPS measurement is "legally traceable" if: (i) it is carried out using the various test/calibration procedures as required by the Guidelines, and; (ii) the survey has followed the "recommended practices for field and office procedures" as described in the Guidelines.

A GPS system testing/calibration program is considered a prerequisite for demonstrating "competence" and for assuring that GPS-derived coordinates are of a uniformly high quality. Recommendations were made concerning three tests: (a) a zero-baseline test, (b) calibration of the GPS equipment on an existing EDM baseline, and (c) connections to several existing first order geodetic GPS control stations.

Zero-Baseline Test A zero-baseline test is performed to determine the correct operation of a pair of GPS receivers, associated antennas and cabling, and data processing software. The test is carried out by connecting two GPS receivers to a single antenna, using an antenna splitter appropriate for the brand of receiver/antenna. This is a comparatively simple test that can verify the precision of the receiver measurements, as well as validate the data processing software. The Guidelines are not too prescriptive with regards to the frequency of such tests, simply requiring the test should be performed at regular intervals, or before any GPS cadastral survey activity is carried out. The derived slope distance should be less than 3mm.

EDM Baseline Test An EDM baseline test is performed in order to ensure that the operation of a pair of GPS receivers, associated antennas and cabling, and data processing software, give distance results that can be compared with calibrated baseline data. EDM calibration baselines have been established throughout Malaysia to service the land surveying community. These baselines have been calibrated against a "standard", and hence satisfy the requirement of "legal traceability". There are a number of difficulties however. GPS can be used to measure the three components of a baseline, but EDM baseline testing only checks the distance component. In addition, EDM baselines are rarely longer than one kilometre, well short of the baseline length "range" over which GPS can operate. Furthermore, many existing baselines do not have good (i.e. clear) sky coverage. (The DSMM does intend to remedy this situation.) As the calibration of EDM is normal practice, it was considered that it would not be too onerous a requirement to also undertake such a test should be performed on a three monthly basis or prior to any large survey campaign being carried out. The resulting difference in slope distance between the GPS measurement and the baseline standard must be less than 10mm. If this tolerance is not met the test should be repeated, and if the equipment fails again the instrument should be returned to the GPS agent for repair.

GPS Network Test The purpose of the GPS network test is to compare GPS-derived coordinates with their corresponding established GPS geodetic values. Such a test is the most complete (and realistic) of the tests, but it is the most complex to administer. Checking the relative coordinates of several control points of the DSMM's national GPS Network (and proposed 5km densification of the same) is one option. The other would be to establish a dedicated test network across which all testing must be carried out. It was decided to adopt the former. In addition, unlike the other tests which were carried out as specific tasks, the 'network test' was developed in such a way that it was part of the standard GPS cadastral survey. In the Malaysian context, it was decided that the GPS survey must be connected to at least two points of the DSMM's network so that starting coordinates (in WGS84) are obtained and to facilitate the determination of the transformation between the WGS84 datum (in which the GPS results are expressed) and the Cassini Soldner (state-level) reference system in which the cadastral survey points are expressed (via intermediate transformations that involve the local geodetic datum known as the Malaysian Revised Triangulation (MRT) System and the Rectified Skew Orthomorphic (RSO) Projection System). It was decided that the maximum allowable discrepancy between the surveyed coordinates (observed GPS values) and the true coordinates (established values) for the network test must be less than 10mm in the horizontal component.

GPS Cadastral Survey: The recommended practices as far as the network test, as well as the guidelines for using GPS to connect the cadastral marks to the surrounding cadastral and geodetic control, are best illustrated using an example. A sample GPS cadastral survey was carried out on several parcels in the State of Melaka. The area is chosen close to the existing Cadastral Standard Traverse along the Melaka-Johor state border (Figure 1). The survey area comprised of six cadastral lots. The area is agricultural land of paddy field where the sky clearance is good enough for GPS survey.

MC793 and KGPB are part of the existing Cadastral Standard Traverse stations which are also connected to the National First Order GPS Network. As they were more than 10km away from the site of the cadastral survey, the guidelines recommend that the GPS control must be 'brought in' so that GPS can be used to survey parcel points using the rapid static technique. Hence two nearby standard traverse stations, namely MC858 and MC904, was selected to be connected to MC793 and KGPB using the static GPS technique.

GPS Cadastral Control Survey GPS observation for establishing the control was carried out by connecting the DSMM GPS Network points MC793 and KGPB, to the existing cadastral control stations (coordinates in Cassini Soldner System) MC858 and MC904. A network of four stations are observed using static GPS (1 hour session). (If there are insufficient GPS receivers, this survey may be carried out over several sessions, and the baselines adjusted using least-squares network adjustment software.) The resulting coordinates are then transformed to their corresponding values in the local RSO and Cassini Soldner system.

Cadastral Survey with GPS The cadastral survey of the parcels (see Figure 2) would then be carried out using the rapid static GPS technique. Two base stations have been used to provide independent checks (redundant baselines) on the resulting GPS coordinates for each boundary marker. Two sets of GPS coordinates for the 14 boundary markers are first computed in the WGS84 datum. The first set is referenced to the GPS base station at MC858 (Base I), and the second set is referenced to the GPS base station MC904 (Base II). The coordinates are then transformed into their corresponding values in the local Cassini Soldner system (WGS84->MRT->RSO->Cassini) using established procedures. The survey is deemed to have been satisfactorily carried out if the difference between the two sets of coordinates are less than 2cm (3-D).

The GPS survey of the boundary markers of the cadastral lots has been carried out following the "recommended practices for field and office procedures" given in the Guidelines. The combination of recommended testing procedures and the recommended survey practices is considered sufficient to ensure that high quality GPS-derived cadastral coordinates can be obtained on a routine basis.