boundaries

How to determine the boundaries of a locality

Q: In beheshti zewar, in the chapter of salaah on a journey, mention is made that if the azaan from one place cannot be heard in another place, then they are regarded as two places, for the purpose of the musafir if he spends 10 days in each place, he would still be a musafir. Does this still apply today when determining where are boundaries? Or does one look at the municipal boundaries between suburbs and localities?