What's an isogloss?
Linguists plot out dialect map into more detailed isoglosses, which are boundary lines between places or regions that differ in a particular linguistic feature. Maps of the dialects of the United States (and other countries) can also be constructed based on the ways in which particular sounds are produced.
The process of drawing a regional dialect map consists of collecting samples of the way people call certain objects, usually object used everyday or found in nature. A group of people have the same dialect if they share many of the words for things. Here is an example of an isogloss, for the regional words for "dragonfly":
