Thailand National Transformation Parameter
Geodetic Parameter used in Thailand:
World Geodetic System (WGS) 1984 Parameters
Datum:WGS84 (BE)
Spheroid:WGS84
Semi Major Axis:a = 6378137.000 m
Inverse Flattening:1/f = 298.257223563
Universal Transverse Mercator (UTM) Indian1975 Parameters
Datum:Indian 1975
Spheroid:Everest 1830
Semi Major Axis:a = 377276.345 m
Inverse Flattening:1/f = 300.8017
UTM Zones:47N, 48N
Central Meridian:99 E, 105 E
Scale Factor at Central Meridian:0.9996
Transformation Parameter from WGS84 to UTM (Indian 1975)
The following two government agencies are using different parameters.
- Department of Land (DOL), Ministry of Interior.
- Royal Thai Survey Department (RTSD), Ministry of Defense.
Royal Thai Survey Department (RTSD) is adopting 3 parameter transformations as follows:
Three Parameter Transformation from WGS84 to Indian 1975
dX=-206 m
dY=-837 m
dZ=-295 m
RTSD is the national mapping agency of Thailand, which is responsible for the entire small scale mapping of the country. They are using this 3 parameter for the entire country. The RTSD data has horizontal and vertical coordinate. The elevations are referred to ellipsoidal height.
Some of the RTSD GPS stations have been connected from the nearest national Bench Marks thus giving mean sea level height
It is said that Dr Chukiat Nichaencharoen, head of the Survey Department, Chulalonkarn University was the consultant for the RTSD and he was involved in the computation of transformation parameter. During the computation he had computed the 7 parameters but the rotation and scale was found to be very small and insignificant. Thus it was decided to adopt only 3 parameters for the transformation.
Though this 3 parameter does not allow for rotation and scale, it has been widely used. Most of the Consulting Engineering companies are utilizing this parameter to transfer their GPS data into Local system.
Department of Land (DOL) is adopting 7 parameter transformations as follows:
Seven Parameter Transformation from WGS84 to Indian 1975
dX=-293.725 m
dY=-836.618 m
dZ= -318.787 m
rX=-0.519"
rY=-1.625"
rZ= 2.825"
dS=-2.055 ppm
DOL has determined this parameter as part of World Bank sponsored “Land Titling Project in Thailand”. This parameter has adopted rotation convention as in Australia, because the consultant was Australian (BHP Engineering) and it was funded by Australia. Though they are using 7 parameter DOL has given only horizontal coordinates for their monument. DOL is not using elevation for their work that may be the reason why they exclude height for their stations.
I suppose they have utilized the RTSD control stations to derive this parameter which caters for the rotation and scale.
DOL has been establishing and densifying their GPS control down to Changwat and Tamboon level by establishing a huge monument with marble facade including engraved coordinate for their Land Titling project. This huge monument stands out quite well but one need the special adopter to set the equipment on to the mark thus barring the other users than the DOL.
It is said that DOL will be utilizing 3 parameters instead of 7 parameters for their future work. For this they are in the process of deriving the 3 parameters to be used.
As far to my knowledge these are the only two Official Parameters used in Thailand for the transformation of WGS84 into local coordinate system.
The published research papers in 1991 in “GPS world” titled “ Establishing SPS Survey Control an Transformation Parameters in Thailand” by Rod Eckels and Khun Pairoj Pheukvilai (DOL), has defined a new parameter which is similar to 7 parameter used in DOL. The figures derived here is some what different from DOL figure.
The other paper published on the web at titled “Realization of the ITRF94 in South East Asia by use of a combined network for geodynamics and national survey in Thailand and Malaysia” by 3 Germans from BKG from Frankfurt, S. Mingsamon and C. Boonphakdee from RTSD and S. Abu from Dept of Surveying and Mapping, Kuala Lumpur, has defined a different set of parameters which is similar to RTSD parameter but with 7 parameter.
It is seen that different transformation parameters are being defined and used by different major projects like Thai Shell, BP, Unocal etc. for their own use
Following is the comparison of the parameters.
Transformation Parameter from WGS84 - Indian 1975 defined by different sourcesParameter / Unit / RTSD / DOL / Article from GPS World (1991) / Article “Realization of ITRF94 in South East Asia...”
dX / m / -206.000 / -293.725 / -293.000 / -203.884
dY / m / -837.000 / -836.618 / -836.000 / -832.904
dZ / m / -295.000 / -318.787 / -318.000 / -293.969
rX / " / -0.519 / -0.500 / -0.0038
rY / " / -1.625 / -1.600 / -0.0579
rZ / " / 2.825 / 2.800 / 0.0781
dS / ppm / -2.055 / -2.100 / -0.017
There are several GPS stations from RTSD and DOL which we can get both WGS84 and Local coordinates. Normally we are using RTSD GPS stations as a control for our establishment of GPS control for the various projects.
New Geodetic System:
This problem of Transformation Parameter has created confusions and inaccuracies in the transformed local coordinates, to overcome this situation and to have a transformation free conversion Thailand has adopted the new Geodetic System based of WGS84 ellipsoid as follows.
Ellipsoid: WGS84
Semi Major Axis:a = 6378137.000 m
Flattening:1/f = 298.257223563
Projection:UTM
Scale factor at Central Meridian:0.9996
Royal Thai Survey Department (RTSD), responsible for the national network all over Thailand is engaged in the transformation of the old coordinates into the new system. All the topographic maps (1:50,000) are being reproduced in this WGS84 UTM Projection system. This new system will be utilized for all the future projects.
Nara Shrestha
Survey Consultant
C B Surveying Co Ltd
Email:
18 February 2004
Sample of photographs of the DOL GPS Station
Sample of the Description Card of DOL GPS Station
Sample of the Description Card of DOL GPS Station
Photgraphs of RTSD GPS Station
Sample of the description of RTSD GPS station
Thailand National Transformation Parameter.doc1 of 911 May 2004