Eiruvin 5:5-6

Eiruvin 5:5

Only a skilled surveyor may measure the Shabbos boundary. If one corner of the boundary is found to be longer than the other, they set the boundary according to the longer side. If two experts measured the boundary and reached different conclusions, they follow the larger measurement. Anyone is believed to report where the Shabbos boundary ends, including a male or a female non-Jewish servant. Since it is a rabbinic enactment, the Sages were lenient when it comes to setting the limits of the Shabbos boundary.

Eiruvin 5:6                                                                                  

If a town was owned by a private individual but it came to be owned by many people, they may make one eiruv to enable all of them to carry (as was the case when it was privately owned). If a town was owned by many people but it came to be owned by a private individual, they may not make one eiruv for everyone unless the owner made the outside of it the size of the town on Chadasha in Judea, which had fifty residents. This is the opinion of Rabbi Yehuda. Rabbi Shimon says it is sufficient if the space outside the city is big enough for three courtyards containing two houses each.