Triangulation and GPS Networks
Employing GPS surveying requires well-established regional and global geodetic networks. They are essential for the preparation of consistent regional and global spatial data. In our case, the Malaysian Datum (West Malaysia), adopts the Modified Everest ellipsoid as a reference with its origin fixed at Kertau, Pahang. The first national geodetic network known as Malayan Revised Triangulation (MRT) consisted of 70 trigonometrical points which is geometrically connected by 340 observed angles and 2 geoidal distances. The MRT network serves as a main control for surveying and mapping activities in our country, including cadastral survey (e.g. standard cadastral traverses). Unfortunately, some of the triangulation points have been 'destroyed' and/or not well maintained. Thus, for the past 15 years, in conjuction with GPS campaign, the re-establishment and updating of the triangulation trig points have been greatly demanded in Malaysia. The Malaysian Datum is different from WGS84 that GPS employs as the reference frame. While the request for the coordinates referenced to the existing datum are still large, demands for the well-accepted global coordinate systems have been also growing. Therefore, a move to adopt geocentric datum has been underway in various country, including Malaysia because it can provide unified geographical reference frame, Teng, (2001).
In Malaysia, serious precise surveying by GPS has started with the establishment the national 1st. order GPS network in 1992, providing a consistence set of coordinates in WGS84. The network is consists of 238 GPS stations (and 171 stations in East Malaysia) which formed the backbone of the National GPS Network. Since then, GPS surveying has been practiced for various surveying and mapping activities in the country. Current trend indicates that for the most precise, effective, economical and fast applications, some form of permanent GPS network should be established. In order to realize such requirements, DSMM has started establishing permanent GPS tracking stations or Malaysian Active Satellite System (MASS) at the end of 1998. Currently, MASS network is consists of 17 permanent GPS stations over the country for scientific research purposes, (e.g. crustal movement, unified datum, earth rotation parameters, atmospheric studies, etc. This network is known as the Zero Order Geodetic Network and it complies with international standards to provide the highest precision for positioning in Malaysia. The National 1st Order GPS Network has been successfully connected to MASS network and its coordinates referred to the ITRF2000 Epoch 00.0 with an accuracy of 1 to 3 cm. Thus, it will form the backbone for the national adjustment of the existing GPS stations to defined all coordinates in ITRF system. By doing this, the adoption of geocentric datum will definitely lead to a homogeneous national coordinate datum across the country (including East Malaysia). Subsequently, in order to benefit directly from proposed geocentric datum, a new projection (and associated parameters) has to be developed so as to allow the coordinates to be projected directly from control network snd dstum to the plane grid system. However, the new projection should have the characteristics of conformal and also, at the sane time maintaining minimal scale distortion.
Coordinate system and transformation
The national (local) geodetic coordinate system consists, in principle ellipsoidal coordinates at a defined set of points. In practice, the Kertau Datum of the MRT, is defined in terms of only the horizontal components (f, l) of the Modified Everest Ellipsoidal coordinates: there is no explicit definition of the ellipsoidal height, h. Now, the first and fundamental answer that any GPS users requires is the question where am I on the Malaysian topographic map at scale 1:50,000?". The GPS receiver will yield geocentric coordinate system in the reference frame to which the satellite orbits are referred. It will not produce coordinates in the local mapping system. If geocentric GPS coordinates are to be very useful for local system, there has to be a coordinate transformation process (function, algorithms) whose form will depend on local information about the mapping control system and the shape of the geoid. In our case, datum transformation enables us to combine GPS measurements with the existing conventional terrestrial measurements whereby 32 triangulation points (MRT system) are used to determine 6 transformation parameters between MRT and WGS84 system. The parameters are estimated by least squares methods using model Bursa-Wolf transformation model - see Table: 1.
Table 1: Transformation Coordjnate from Geocentric WGS84 to MRT Kertau
| Items |
Transformation Parametes |
| Dx |
379.77603 m |
| Dy |
-775.38371 m |
| Dz |
86.60926 m |
| Rx |
2".59674 |
| Ry |
2".10213 |
| Rz |
-12".11377 |
The existing coordinate systems used for mapping and cadastral survey in West Malaysia is the conformal Rectified Skew Orthomorphic system (RSO) and Cassini Soldner system (CS), respectively. The RSO projection system is also based on the Modified Everest reference ellipsoid while the Cassini Soldner is a plane coordinate system for local cadastral system. The mathematical projection models and associated coefficients for RSO and Cassini coordinate systems can be found in Hotine (1947) and in Richardus and Alber, (1974), respectively. A number of origins have been adopted when establishing local Cassini coordinate system, resulting in each state in West Malaysia using a difference origin.. Since the adjusted GPS coordinates are in geocentric datum, i.e. WGS84, they are need to be transformed into these established local systems. The transformation procedure from global datum to local datum (mapping and cadastral system) involves a lengthy computation steps as followed:
- WGS84 to MRT : (f, l, h) global => (f, l, h) local
- MRT to RSO : (f, l, h) local => (x, y) local
- RSO to CS : (x, y) local => (E,N) local
It should be noted here that the difference system between the RSO system (mapping) and the CS system (cadastral) has resulted in incompatibility as far as digital database is concerned. One of the main objective of the CCS is to study the feasibility of adopting the earth-centred geodetic datum RSO as a nationwide coordinate system for cadastral survey. It will replace the currently anachrosnistic Cassini system for cadastral survey. The concept and the stages of implementation of a 'geocentric' RSO projection system for the whole country, i.e. will not included in this paper.