Eruvin 59A

גמ׳ למקום שריבה אין, למקום שמיעט לא? אימא: אף למקום שריבה.

Talmud: The Talmud asks: Does this mean that in a place where he extended the limit, yes, the surveyor’s measurements are accepted, but in a place where he reduced the limit, no, his measurements are not accepted? If his extended measurement is accepted, his shortened measurement should certainly be accepted as well. The Talmud answers: Say that the Mishnah means that the surveyor’s measurements are accepted even in a place where he extended the limit, without concern that he might have erred (Tosafot), and that the surveyor’s measurements are certainly accepted in places where he reduced the Shabbat limit.

ריבה לאחד ומיעט לאחד כו׳. הא תו למה לי? היינו הך! הכי קאמר: ריבה אחד ומיעט אחד ⁠— שומעין לזה שריבה.

We learned in the Mishnah: If the surveyor extended the limit for one and reduced it for another, one accepts the extended measurement. The Talmud asks: Why do I need this as well? This clause is the same as that previous clause in the Mishnah. The Talmud answers that this is what the Mishnah said: If two surveyors measured the Shabbat limit and one extended the Shabbat limit and one reduced it, one accepts the measurements of the surveyor who extended it.

אמר אביי: ובלבד שלא ירבה יותר ממדת העיר באלכסונא.

Abaye said: The measurements of the surveyor who extended the limit are accepted only as long as he does not extend the limit more than the difference between the measure of the Shabbat limit of the city calculated as a diagonal line from the corner of the city and as calculated as a straight line from the side of the city. If, however, the difference in measurements exceeds that amount, the Shabbat limit must be measured again.