Original property survey vs new property survey

We live in an older section of Sacramento. We just had a licensed surveyor survey our property line between us and one of our neighbors. He discovered that the property line is closer to our house than originally thought; in fact, part of our original eave is located about 4 inches onto our neighbors’ property, according to the new survey. Our house was built in 1913, a year after the area was subdivided into parcels. We are assuming that our house was built using the property line set in 1912 (why would the city allow them to encroach on the adjacent property). We want to build a fence at the legal property line. Considering that this is the first survey of the property line since 1912, and that property taxes have been paid on the land and structure since then, which is the legal property line?