FYI I believe C.C. Cheng used GPS for a house-by-house study of variation in his home town in Taiwan a few years ago. Cheng, Chin-Chuan. 2004. A Microscopic View of Geographical Distribution of Southern Min and Kejia Dialects in Taiwan http://www.ling.sinica.edu.tw/english/v3-3-2.asp-UID=42&v_type=1.htm