GPS and Thai UTM coordinates

Whenever in Royal Gazette announcements geographical locations like boundaries of the administrative subdivisions are mentioned, these are given in a special coding not directly understandable. Few months ago I cracked the code, when in the announcement of a municipal boundary change I understood that e.g. VE ๒๐๔๕๐๘ means the UTM coordinate 48Q4204 19508. However converting the UTM coordinate into longitude and latitude it did not fully fit with the place marked in the map enclosed in the announcement.

A year ago I found Excel sheets at the DOPA website which asked the local administrations to check the names of the administrative villages (Muban) and fill in the coordinates in UTM notation. With the first filled sheets now available I now had a lot of UTM coordinates to be parsed, and therefore had to look for code to convert them into "normal" geographic coordinates. I not only found that, but also learned a whole new thing about geographic coordinates, that additionally to the two coordinates one has to give the datum, which mostly covers the parametrization of the flatness of the earth globe. Google Earth as well as GPS uses the WGS84 datum, however in Thailand coordinates most usually are given in the Indian 1975 datum, including those UTM coordinates for the Muban. The two coordinates differ by about 80 metres, still enough to make a pointer to a specific building point to a totally different one, especially in more densely settled areas. The different datums even led to a serious misalignment of various layers in Google Maps, luckily not for Thailand.

Yet, not all instances use the Indian 1975 datum. A marker stone I found between the provincial hall and the provincial court of Surat Thani has the exact geographic location of that place, given in the WGS84 standard retrieved with the GPS system. That stone inscription reads 9° 7' 58.32484'' N 99° 19' 53.9080'' E, assuming this value is correct it means the Google Earth imaginary of Surat Thani is misaligned by about 2 metres. But I somehow doubt the accuracy given in the inscription - 0.0001 arcseconds means about 3 millimetres, way beyond the accuracy possible with GPS. IMHO at least two digits of the numbers are simply bogus.