Eruvin, Chapter 5, Mishnah 5. The Mishnah continues with the details of measuring the Shabbos boundary from the city: how we establish the marker for the point where the boundary ends.
"Ein moddin ela min ha'mumcheh" - we measure only by way of an expert:
We do not establish the boundary markers based on the measurement of just anyone, but only on that of an expert. He alone is the one we rely upon to determine where the boundary ends.
He measured more in one place and less in another:
The measurement was taken on one side of the city, and it turns out that in one place the distance came out longer than in another, and there is no way to account for this other than an error made in one of the two places. That is, we are not dealing with a city that extends further on one side than on the other - the line is straight - and nevertheless, when they measured from one spot two thousand cubits came out to here, and when they measured from another spot two thousand cubits came out to there. In such a case we follow the place where the larger measurement came out, since in everything having to do with eruvin and eruv techumin we rule leniently.
The same applies when two experts measured: one fixed a more distant point and claimed that this was the two thousand cubit measure, and the other fixed a nearer point - we listen to the one who measured the greater distance.
The credibility of a slave and a maidservant:
Even a slave and even a maidservant, who are only partially Jewish, "ne'emanim lomar: ad kan techum Shabbos" - are believed to say: up to here is the Shabbos boundary.
The reason for these leniencies:
The reason for all these leniencies is that the Sages did not state the laws of Shabbos boundaries in a way that would require us to be stringent about them. The law of techumin is rabbinic, and we hold to the principle that a doubt in a rabbinic law is resolved leniently. All of these cases are nothing more than doubts, and therefore the boundaries are not established according to the stringent view but according to the lenient one, and for that reason we are lenient in all of them.
In summary: In this Mishnah we learned that the boundary is measured only by an expert; that where two different measurements came out, due to an error in the measuring, we follow the larger measurement - both when he measured more in one place and less in another, and when two experts disagreed; and that even a slave and a maidservant are believed to say that up to here is the Shabbos boundary. And the reason is this: the laws of techumin are rabbinic, and a doubt in a rabbinic law is resolved leniently, for the Sages did not state the matter in order to be stringent but in order to be lenient.