Rif Eruvin 17a {Eruvin 58b}

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17a
{Eruvin 58b}
But, if a plumb-line could descend exactly {to the floor of the valley}, he should measure it a precise measure.

And how much is the depth of a valley {in order that it may be passed over}?
Rav Yosef said: 2000 {cubits}.
Abaye asked {from a brayta}: If it is 100 cubits deep and 50 cubits wide, he spans it, and if not, he may not span it.
This is in accordance with the "Others." For they learnt {in a brayta}: Others say: even if is 2000 cubits deep and 50 cubits wide, he spans it.

Another version has it:
Rav Yosef said: Even more than 2000.
Like whom? This is not like the Sages and not like the "Others." Here {not > 2000} it where the plumb-line cannot descend straight down, and here is where the plumb-line can descend straight down {and thus it is difficult to descend - R Yehonatan}.

And if the plumb-line can descend straight down - until how much {may the slope be to be reckoned so}? Abaye said, and so did Rami bar Yechezkel teach: 4 cubits. {But see R Yehonatan.}

"If he reaches a mountain {hillock}, he absorbs {spans} it and returns to his measure":
Rav Huna the son of Rav Natan said: They only said this regarding a mountain {has the slope such that} it reaches a height of 10 handbreadths within 4 cubits, but a mountain that reaches a height of 10 handbreadths within 5 cubits, he estimates it and continues {measuring}.
And so is the halacha.

"provided he does not go out of the techum":
What is the reason?
Rav Huna said: As a decree lest they say that the techum measurement reached here.
To explain: So that those who witness {his measurement} do not err and say that the techum measurement reached here.

"and if he cannot span it":
How should he "pierce" it?
They taught {tana}: The lower one {holds the rope} next to his heart and the upper one by his feet.
Abaye said: We have a tradition that we only "pierce" with a rope measuring 4 cubits.
Rav Nachman cited Rabba bar Avuah: We do not "pierce," not by {establishing the techum for} cities of refuge {for accidental murderers} nor by the heifer which has its neck broken {when a victim of murder is found between two cities}, for they are Biblical.

Mishna:
They may not measure except by an expert. If it increased in one place and diminished in another place {in two separate measurements, from different corners}, they heed the place that increased {assuming that for the shorter measurement, the rope had not stretched out sufficiently}.
If it increased for one {expert} and diminished for another {expert}, they heed the increased.