Maps are intended to help you understand a place. Sometimes they appear to answer your questions but remember that all maps contain errors. Details meant to be viewed at a city scale are probably not accurate when zoomed to the corner of your front yard.
Property lines, in particular, should not be used for boundary determination, conveyances, or accurate measurement purposes - authoritative property boundaries require the services of a professional land surveyor.
Map data are gathered from many sources: some are "live," some are updated periodically -- it's only a snapshot of information that is constantly changing. If your needs are exacting please contact the appropriate department for more detailed and current information.
City Authoritative Data Sources:
Assessors (property ownership) at 413-587-1269
Public Works (City infrastructure) at 413-587-1570
DigSafe (non-City infrastructure) at 888-344-7233
Planning and Sustainability (parcels and demographic data) at 413-587-1262