Change of Map Datums – The Switch to WGS 1984

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It was inevitable. At some point I was going to have to change to the WGS 1984 map datum from the Indian 1954 (Thailand-Vietnam) datum that has been used by the Thailand Caves Database. This procress has now been started, but it will be a long job and for the next couple of years there will be a mix of datums used on this site.

The Indian 1954 (Thailand-Vietnam), or the very similar Indian 1975 datum, was used by the Royal Thai Survey Department (RTSD) for their 1:250,000 and L7017 series 1:50,000 maps as well as for historic maps produced by the RTSD and US military. However, since the appearance of GPS in the late 1990s many national survey departments have been changing to the global WGS 1984 datum to make their maps GPS compatable. The RTSD began this process in the mid 2000s and have started to release to the public the new L7018 series 1:50,000 maps on the WGS 1984 datum. These new maps have information to 2007 and have been completely redrawn. Compared to the old L7017 series the road and village locations on the L7018 maps are a lot more accurate and detailed. A new feature on the L7018, which is of interest to cavers, is that disappearing karst streams are clearly marked with a blue dot.
 
WGS 1984 is also the datum used by Google Earth, Google Maps and car navigation GPS units such as the Garmin Nuvi. However, I will continue to use UTM grid co-ordinates. This is because it is easier to work out the distance between two points when the co-ordinates are UTM, compared to using degrees and minutes.
 
As there are approximately 3,000 co-ordinates in the Database it is not a quick task to convert to the new datum. The change will be made province by province as the caving guides are compiled or updated. The WGS 1984 datum has so far been used in the Phuket and Kamphaeng Phet guides.