883. Approximating Mountains and Valleys

Shabbos 28:16

Approximating measures for mountains and valleys that cannot be spanned is done as follows: two people hold a rope that is four cubits long (about 6'). The one above holds the higher end of the rope by his feet and the one below holds the lower end of the rope by his heart. The higher person descends to the level of the lower person, who then descends further to the length of the rope. This process is repeated until the entire area is measured. When surveyors go to measure a mountain or a valley, they do not depart from the Shabbos boundary because passersby might see them and think that the boundary includes that area.

Shabbos 28:17

We rely only on measurements taken by an expert surveyor. If a Shabbos boundary is already established and an expert re-measures it, increasing it in some places and decreasing it in others, we accept his opinion regarding the areas that he has increased. Similarly, if two experts measure the Shabbos boundary and one gives a larger measure than the other, we accept the opinion of the one who gave the larger measure so long as the difference between them is not larger than the difference between the diagonal and one of the sides of the imaginary box drawn around the city.