I assume that in Thailand or Myanmar, bilateral state treaties are made public in some form.
Not certain whether they are available online.
You should be able to look them up in some government agency and probably also be able to make copies (potentially for a small fee).
This is currently out of scope for my language level of Thai. Others might be able to come up with documents. Maybe Mishari knows where to search, as he worked with some of the open-data before.
Here you can read up how other countries deal with border definitions, for example the longest land border:
https://en.wikipedia.org/wiki/Canada%E2%80%93United_States_border
As you can see, even they have disputed areas, which is then another obstacle you might face. There is a specific tagging on how to mark disputed borders.