We may not measure except with a fifty cubit rope, neither shorter nor longer. One may not measure except [with the rope] opposite his heart. If he was measuring and reached a valley or a mound, he should span it and return to his measuring; if he reached a mountain, he should span it and return to his measuring; provided he does not go out of the techum. If he is unable to span it — regarding this R’ Dostai bar R’ Yannai stated in R’ Meir’s name: I have heard that we pierce mountains.