Reference for Eruvin 115:14

אם היה גיא מעוקם מקדיר ועולה מקדיר ויורד הגיע לכותל אין אומרים יקוב הכותל אלא אומדו והולך לו

that is in a straight line with his original line of measuring<span class="x" onmousemove="('comment',' Cf. relevant notes on our Mishnah and first diagram ibid. Lit., 'corresponding to his measure'.');">²⁷</span> and then he resumes [his measurements in a straight line] - Thus we have here learnt what the Rabbis have taught elsewhere: If in the course of measuring the measuring rope reached a glen, the surveyor may span it if he can do so with a rope of fifty cubits, but if not, he proceeds to a position<span class="x" onmousemove="('comment',' Away from the town.');">²³</span> from where<span class="x" onmousemove="('comment',' The width across being less than fifty cubits.');">²⁴</span>

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