If you have ever tried to overlay an old Malaysian topographic map onto a modern GPS device or a web mapping application, you may have noticed that features do not quite align. Rivers appear slightly shifted. Hilltops sit a few hundred metres from where the map says they should be. This is not a printing error or a GPS glitch. It is the result of a fundamental shift in the way Malaysia defines the position of points on the Earth’s surface.

To understand why this happens, you need to understand four interconnected concepts: the datum, the ellipsoid, the map projection, and the coordinate reference system. This article walks through each of these in plain language, explains the historical context of the Kertau system, describes the RSO Malayan map projection, and then covers both GDM2000 and its successor GDM2020: Malaysia’s current geodetic standard.

The Problem With Mapping a Round Earth

The Earth is not a perfect sphere. It bulges slightly at the equator and is flattened at the poles, making it an oblate spheroid. For mapping purposes, geodesists approximate the true shape of the Earth using a mathematical surface called an ellipsoid (sometimes called a spheroid). An ellipsoid is defined by two parameters: the semi-major axis (the equatorial radius) and the flattening ratio.

Different ellipsoids have been defined over the centuries as measurement technology improved. Some were designed to best fit the whole Earth; others were optimised to fit a particular region more accurately. The choice of ellipsoid matters because it directly affects the coordinates assigned to any point on the ground.

A geodetic datum goes one step further. A datum combines an ellipsoid with a defined origin point (or in modern systems, a set of control stations), and it specifies exactly how the ellipsoid is oriented and positioned relative to the physical Earth. Two points at the same physical location can have different latitude and longitude values if they are referenced to different datums. This is the root cause of the alignment problem described above.

The Kertau Datum: Malaysia’s Historical Reference System

Origins and History

The Kertau 1948 datum, officially known as the Malayan Revised Triangulation 1948 (MRT48), was established by British colonial surveyors in the mid-twentieth century. The datum takes its name from Bukit Kertau, a hill in Pahang that served as the origin point of the triangulation network for Peninsular Malaysia.

The ellipsoid used for the Kertau datum is the Modified Everest 1830, a regional ellipsoid originally computed by Sir George Everest for the Great Trigonometrical Survey of India. The Modified Everest ellipsoid was later refined specifically for use in the Malay Peninsula. It was not designed to fit the global Earth, but rather to give the best local fit over the terrain of Peninsular Malaysia. As a result, coordinates derived from the Kertau datum can differ from modern GPS coordinates (which use a globally fitted ellipsoid) by anywhere from a few metres to over 200 metres in certain areas of the peninsula.

The Kertau datum was used as the basis for all official topographic mapping of Peninsular Malaysia from the 1940s until the adoption of GDM2000 in the early 2000s. The iconic 1:50,000 topographic series published by Jabatan Ukur dan Pemetaan Malaysia (JUPEM) was produced on this datum. Sabah and Sarawak used separate datums: the Timbalai 1948 datum for Sabah and parts of Sarawak, also based on a Modified Everest ellipsoid but with a different origin point.

Why a Local Datum Was Necessary

Before GPS and satellite geodesy, establishing the position of a point on the Earth’s surface required a long chain of triangulation measurements, starting from a carefully determined origin point. The accuracy of the entire network depended on the precision of the initial measurement and the quality of the ellipsoid fit in the survey region. Using a globally optimised ellipsoid would have introduced small but systematic errors across the survey area. A locally fitted ellipsoid minimised these errors, making the resulting maps more internally consistent, even if they did not tie perfectly to coordinates measured elsewhere in the world.

This trade-off was perfectly acceptable when maps were used on their own. The problem arose decades later, when GPS receivers became widely available and users began expecting their device coordinates to match what was printed on the map.

Grid Terunjur: Turning the Sphere Into a Flat Sheet

What Is a Map Projection?

A map projection is a mathematical method for transforming the curved surface of the Earth onto a flat plane. No projection can do this without introducing some form of distortion. Depending on the projection chosen, the map may distort distances, areas, angles, or some combination of all three. Different projections are selected for different purposes based on which properties are most important to preserve.

The term Grid Terunjur in Bahasa Malaysia translates roughly as “projected grid” and refers to any coordinate system that results from applying a map projection. When you see easting and northing values on a Malaysian map (typically expressed in metres), you are reading a Grid Terunjur coordinate.

The RSO Malayan Projection

The map projection used with the Kertau datum for Peninsular Malaysia is the Rectified Skew Orthomorphic (RSO) Malayan projection, formally identified by the EPSG code EPSG:3168. This is a conformal (angle-preserving) cylindrical projection based on the Hotine Oblique Mercator family.

The standard Mercator projection works well for areas that are oriented north-south or that span a wide range of latitudes near the equator. However, the Malay Peninsula extends roughly from northwest to southeast, and its longest dimension runs diagonally rather than along a meridian. A standard Mercator projection would introduce excessive distortion along this diagonal axis.

The RSO projection solves this by tilting the central projection cylinder so that its axis aligns with the natural orientation of the peninsula, approximately along the northwest-to-southeast axis. The “rectified” part of the name refers to a mathematical adjustment applied to the oblique Mercator to make the grid lines run parallel to the cardinal directions while still minimising distortion along the central axis of the peninsula.

The key parameters of the RSO Malayan projection include:

• Ellipsoid: Modified Everest (Kertau)
• Projection type: Hotine Oblique Mercator, variant B
• Azimuth of central line: approximately 323.025771 degrees (roughly northwest)
• Latitude of projection centre: approximately 4 degrees north
• False easting and northing: defined to keep all coordinates positive within the peninsula
• Scale factor: 0.99984 (slightly less than 1, to distribute distortion symmetrically either side of the central axis)

The resulting coordinate system gives easting and northing values in metres, referenced to the Modified Everest ellipsoid on the Kertau datum. These coordinates appear on JUPEM topographic maps and were the standard format for Malaysian land survey data for several decades.

Practical Implications

If you are using mapping software and need to display old JUPEM maps or work with legacy survey data, you will need to specify the coordinate reference system as EPSG:3168 for the RSO Malayan / Kertau system. Many GIS applications such as QGIS, ArcGIS, and command-line tools such as GDAL and OGR support this EPSG code natively.

Converting from RSO Malayan coordinates to WGS84 (the coordinate system used by GPS and most web maps) requires a datum transformation in addition to the projection calculation. The transformation involves applying a set of shift parameters (known as a Helmert or seven-parameter transformation) to account for the difference in ellipsoid size, shape, and positioning between Kertau and WGS84.

GDM2000: Malaysia’s First Modern Geodetic Framework

The Transition to Satellite Geodesy

By the late 1990s, GPS had become the dominant method for determining geographic coordinates. GPS satellites transmit their positions relative to a global reference frame called the World Geodetic System 1984 (WGS84), which is based on a globally fitted ellipsoid known as GRS80. For practical purposes, WGS84 and the global reference frame known as ITRF (International Terrestrial Reference Frame) are equivalent.

The Kertau datum and RSO Malayan projection were not designed with GPS compatibility in mind. Using a GPS receiver in Malaysia and plotting the result on a Kertau-based map would introduce systematic positional errors of 100 to 200 metres. JUPEM recognised the need to bring Malaysian geodesy into alignment with global satellite-based systems.

What Is GDM2000?

GDM2000, which stands for Geodetic Datum Malaysia 2000, is the satellite-compatible geodetic reference framework adopted by Malaysia for Peninsular Malaysia. It was realised through a network of continuously operating reference stations (CORS) known as the MyRTKnet (Malaysia Real-Time Kinematic Network), which was established by JUPEM and progressively expanded from the early 2000s.

GDM2000 is based on the GRS80 ellipsoid and is aligned to the ITRF2000 (International Terrestrial Reference Frame 2000), at epoch 1 January 2000. For most practical purposes, coordinates in GDM2000 are equivalent to WGS84 coordinates within a few centimetres. This means that a GPS coordinate taken with a modern receiver can be directly plotted on a GDM2000-based map without the large datum shift corrections that were required for Kertau-based maps.

GDM2000 as a Static Datum

GDM2000 is a static datum. It assigns fixed coordinates to each CORS station and assumes those positions do not change over time. When the datum was established around 2003, this was a workable approach, and it brought Malaysian geodesy fully into the satellite era. However, a static datum has an inherent vulnerability: the physical Earth continues to move, even when the published coordinates do not.

JUPEM reviewed and revised the GDM2000 coordinate set multiple times (in 2006, 2009, and 2016) to account for accumulated drift. But these were one-off corrections rather than a continuous, self-updating model. The problem became acute following a series of major earthquakes in the Indonesian archipelago.

The Earthquake Problem

Malaysia sits on the Sundaland tectonic block, a relatively stable microplate within Southeast Asia. However, the region is surrounded on almost every side by tectonically active convergent plate boundaries. Major seismic events in neighbouring Indonesia (particularly the catastrophic 2004 Mw 9.2 Sumatra-Andaman earthquake, the 2007 Mw 7.9 Bengkulu earthquake, and the 2012 Mw 8.6 Indian Ocean earthquake) caused measurable physical displacement of the Malaysian landmass and its CORS stations.

Research by Malaysian geodesists found that the cumulative effect of these earthquakes caused the GDM2000 datum to shift by an average of approximately 34.6 centimetres, predominantly toward the southeast. The published coordinates of the CORS stations remained unchanged, but their actual physical positions had moved. For high-precision cadastral surveying and engineering work, this level of discrepancy was no longer acceptable. Something more robust was needed.

GDM2020: Malaysia’s Semi-Kinematic Datum

A New Approach to Geodesy

GDM2020, formally established by JUPEM in October 2021, represents a fundamental change in how Malaysia maintains its national coordinate reference frame. Rather than a static set of coordinates periodically corrected after the fact, GDM2020 is a semi-kinematic datum: one that models the ongoing movement of the Earth’s crust as an intrinsic part of the datum itself.

GDM2020 is aligned to ITRF2014 (International Terrestrial Reference Frame 2014), a more recent and more accurate global reference frame than the ITRF2000 used by GDM2000. The reference epoch is 1 January 2020.

How the Semi-Kinematic Model Works

The key innovation in GDM2020 is that each CORS station in the network has three components assigned to it:

Station position: the precisely determined coordinate of the station at the reference epoch (2020.0).

Station velocity: a vector describing how fast and in which direction the station is moving due to ongoing tectonic plate motion. Because the Sundaland block moves at a measurable rate relative to the ITRF, every station drifts by a predictable amount each year. The velocity model captures this motion.

Post-Seismic Deformation (PSD) model: for stations that were significantly affected by major earthquakes, an additional parametric model describes the irregular, time-decaying deformation that follows a large seismic event. Earthquake-induced ground displacement does not stop when the shaking stops; the crust continues to adjust for months or even years afterwards in a process called post-seismic relaxation. The PSD model accounts for this.

Given any date, the GDM2020 framework can compute the expected position of any CORS station at that moment, accounting for both the steady drift of plate motion and any residual earthquake deformation. This makes it self-correcting over time in a way that GDM2000 could never be.

Why Semi-Kinematic and Not Fully Kinematic?

A fully kinematic datum would assign every user a time-stamped coordinate that changes continuously. While technically precise, this would be operationally difficult for everyday users: surveyors, engineers, and GIS practitioners need stable coordinates they can record in documents, databases, and land title records. The semi-kinematic approach retains the time-dependent modelling internally but outputs coordinates referenced to the fixed epoch of 2020.0, giving users a stable and consistent coordinate framework.

Technical Foundation

The technical basis for GDM2020 was derived from a cumulative solution obtained by stacking the position time series of Malaysian CORS stations collected between 1999 and 2018. Data came from both the MASS (Malaysia Active GPS System) and MyRTKnet networks, covering more than 100 stations across Peninsular Malaysia, Sabah, and Sarawak. The solution achieved internal precision values of approximately 3.0 mm in the east component, 3.2 mm in the north component, and 7.6 mm in the vertical component: a significant improvement over the accuracy achievable with a static datum that had drifted due to seismic events.

The ellipsoid remains the GRS80, unchanged from GDM2000. The improvement comes not from a new mathematical model of the Earth’s shape but from a better-maintained, time-aware realisation of the reference frame.

Impact for Different User Groups

For most everyday users of consumer GPS devices and web mapping applications, the practical difference between GDM2000 and GDM2020 is small: typically less than half a metre across most of Peninsular Malaysia. General navigation and location sharing are not materially affected.

For professional users, the implications are more significant. Surveyors using the MyRTKnet RTK positioning service now receive GDM2020-referenced coordinates. Cadastral plans, subdivision surveys, and engineering setout work should be referenced to the new datum. GIS databases built on GDM2000 will require transformation to GDM2020 to remain consistent with new survey data. JUPEM has published official transformation parameters and software tools to facilitate this migration.

Hydrographic and coastal engineering work also benefits considerably from GDM2020. The datum’s vertical velocity models improve the accuracy of mean sea level determination and vertical land motion analysis, which is increasingly important for flood risk assessment, coastal infrastructure design, and sea level rise monitoring.

Comparing All Three Systems

Why This Matters in Practice

For GIS and Web Mapping

Web mapping platforms such as Google Maps, OpenStreetMap, and Bing Maps all use WGS84-equivalent coordinates. Old Malaysian government spatial data referenced to the Kertau datum must have a datum transformation applied before it can be correctly overlaid with web map tiles. Tools such as QGIS, GDAL, and the Proj coordinate transformation library can perform this transformation, but you need to select the correct parameters to avoid introducing additional errors. Data referenced to GDM2000 overlays with web maps very closely with no additional correction needed. GDM2020 data is equally compatible.

For Amateur Radio Operations

In amateur radio, accurate coordinates are important for antenna pointing calculations, APRS beacon positioning, bearing and distance calculations, and Maidenhead grid locator determination. Applications that import coordinates from old printed maps need to account for the datum. Entering Kertau-based coordinates into a modern radio application expecting WGS84 input will produce a positional error of over 100 metres: acceptable for general HF communication, but significant for precision applications such as satellite tracking, EME (Earth-Moon-Earth) antenna alignment, or microwave path planning.

For Surveyors and Engineers

All professional surveys in Malaysia now reference GDM2020 as the active standard. Legacy boundary data, land titles, and cadastral records referencing Kertau or GDM2000 must be carefully transformed when integrated with new data. JUPEM provides official transformation grids and documentation for both the Kertau-to-GDM2000 and GDM2000-to-GDM2020 transitions.

For Military Personnel

The choice of coordinate datum has direct and serious consequences for military operations. Defence forces rely on precise coordinates for a wide range of activities: navigation in the field, fire support planning, close air support coordination, helicopter landing zone designation, and the programming of precision-guided munitions. A datum error that appears minor on paper can translate into a positional offset of 100 to 200 metres on the ground, which is operationally significant in any of these scenarios.

Angkatan Tentera Malaysia (ATM) has historically used maps produced by JUPEM, most of which were printed on the Kertau datum and the RSO Malayan grid. Field personnel trained to read grid references from these maps must understand that the coordinates they extract are Kertau-based. Entering those coordinates directly into a GPS receiver or a digital mission planning system that expects WGS84 input will place the plotted position in the wrong location. In a benign training context this causes confusion. In an operational context, the consequences can be considerably more serious.

Modern military GNSS receivers and battlefield management systems operate natively in WGS84. As long as field units are working from legacy Kertau-based paper maps alongside WGS84 GPS devices, there is a persistent risk of datum mismatch. Personnel must be trained to recognise which datum a map or coordinate is referenced to before using it in any system, and to apply the appropriate transformation or offset.

Artillery and mortar units are particularly sensitive to coordinate accuracy. A target coordinate error of 100 metres at typical indirect fire ranges can place rounds well outside the intended impact area. Fire missions planned using grid references extracted from old Kertau-based maps and passed without datum conversion to a gun position equipped with WGS84-based ballistic computers will produce exactly this kind of systematic error. Standard operating procedures should explicitly require datum verification as part of the fire mission clearance process.

The transition to GDM2020 also affects military mapping. New digital map products issued by JUPEM reference GDM2020, and coordinates disseminated through modern C4I (Command, Control, Communications, Computers, and Intelligence) systems will increasingly reflect the new datum. Units integrating legacy Kertau or GDM2000 data with GDM2020-referenced systems need to ensure datum transformations are applied at the data integration layer rather than leaving individual operators to reconcile the discrepancy manually.

For special operations and intelligence work, where accurate georeferencing of imagery, signals data, and ground features is essential, datum awareness is equally critical. Imagery from modern satellites and UAV platforms is natively referenced to WGS84 or GDM2020-compatible frames. Overlaying such imagery against legacy map data without applying a datum transformation will produce visible misregistration, potentially leading to incorrect identification of target locations or geographic features.

Summary

The evolution of Malaysian coordinate systems mirrors the global progression of geodesy from classical triangulation to satellite-based positioning and, most recently, to time-aware dynamic reference frames:

• A datum defines the mathematical reference surface (ellipsoid) and its orientation relative to the Earth. Different datums assign different coordinates to the same physical point.
• The Kertau 1948 datum was established by colonial surveyors using the Modified Everest ellipsoid, with Bukit Kertau in Pahang as the origin. It underpinned all official topographic mapping in Peninsular Malaysia for over half a century but is not compatible with GPS.
• The RSO Malayan projection (EPSG:3168) is a Hotine Oblique Mercator projection tilted to align with the diagonal orientation of the Malay Peninsula, converting spherical Kertau coordinates into flat easting-northing metre values: the Grid Terunjur system seen on JUPEM maps.
• GDM2000, introduced around 2003 and based on the GRS80 ellipsoid aligned to ITRF2000, brought Malaysian geodesy into the GPS era. It is a static datum and was revised several times to correct for accumulated drift, but could not inherently account for the crustal displacement caused by major Indonesian earthquakes.
• GDM2020, formalised in October 2021 and aligned to ITRF2014 at epoch 2020.0, is Malaysia’s current national geodetic standard. It is a semi-kinematic datum that incorporates station velocity models and Post-Seismic Deformation parameters, allowing it to remain accurate over time as the Earth’s crust continues to move. It is the reference framework for all new JUPEM surveys and the MyRTKnet positioning service.

Understanding these systems is essential for anyone working with maps, spatial data, or positioning technology in Malaysia. The 100-to-200-metre difference between Kertau and GDM2000 is invisible on a small-scale overview map but is critical in engineering, cadastral, and navigation work. The further evolution from GDM2000 to GDM2020 addresses a subtler but equally important problem: the slow, ongoing drift of the Earth’s crust that accumulates into significant positional errors over years and decades. With GDM2020 in place, Malaysia now has a geodetic framework that is globally interoperable, continuously accurate, and fit for the demands of modern precision geospatial work.

References: JUPEM Technical Publications (2021); EPSG Geodetic Registry (epsg.io); Azhari M. et al. (2020), “Semi-kinematic geodetic reference frame based on the ITRF2014 for Malaysia”, Journal of Geodetic Science; OGP Geomatics Guidance Note 7 Part 2; Hotine, M. (1947), “The Orthomorphic Projection of the Spheroid”; PROJ Coordinate Transformation Library documentation.

The post Malaysian Coordinate Systems: Kertau, Datum, Map Projections, GDM2000, and GDM2020 appeared on Hamradio.my - Amateur Radio, Tech Insights and Product Reviews by 9M2PJU.