This research aims to establish lexical boundaries of the two Thai dialects, Central and Non-central Thai. It was a novel study in Thailand which integrated the technical innovation of the Geographic Information System with the linguistics approach for the overlay analysis of dialect regions. Word data used in this research was taken from the prior work of Word Geography of Thailand Project, comprised of 170 semantic units collected from all across the country in 2002 - 2003 at subdistrict level. Data analysis was done by ways of word classification and grouping through the current techniques in linguistics approach and the spatial overlay of 170 isogloss maps using the Geographic Information System application. Results are in the form of (1) 170 isogloss maps (2) an overlay map created from the bundles of 170 isoglosses and (3) a boundary map of Central and Non-central Thai dialects.

Three findings can be summarized as follows: (1) although the lexical variation of each semantic unit was unique, principle and techniques in geography can be used for the determination of dialect boundaries. (2) The created dialect boundaries were drawn consistently on the basis of district boundaries, separating the areas of people speaking Central Thai between  50% and < 50%. (3) The comparison between the created dialect regions and the geographical regions shows the incomplete coincidence of boundaries in most areas. Furthermore, the by-product of this research shows the current trend and direction of dialect movement as well as its movement compared to the 1988 dialect boundaries of Chalida Rojanawathanavuthi’s research work (1990).