Did you manage to get the projection right? If I remember February right in Thailand is Indian 1975 used.
That’s nearly a kilometer off when not corrected. I found this on the web:
The shift parameters from Indian 1975 to WGS84 have been derived many times. The current ones are:
∆X = +204.4 m, ∆Y = +837.7 m, ∆Z = +294.7 m
(Prof. Itthi Trisirisatayawong, Chulalongkorn University, November 2010)