I agree GPS has been around longer than 2006. But unfortunately my local land office didn't have it then. I picked up the top of the marker for safe keeping after it became detached through weathering and then falling off.
"The Thai title system is based on coordinates." And where are those coordinates kept/stored, along with the land title.? The land office is the primary building followed by your own copy of the title. If you care to look at your land paper, top right hand corner, you find it. Basically the concrete markers are not a definite indication of what's what. The land office is your best bet. Not concrete markers.
Last thing did you get someone to decipher the photo I posted? My missus couldn't so I'd appreciate anyone's assistance in that matter.