882. Measuring Through Walls and Mountains

Shabbos 28:14

When the surveyors reach a wall, it is not necessary to pierce the wall; rather, it is sufficient to approximate the thickness of the wall and to continue on the other side. However, if people use the wall, then it must be measured exactly. Similarly, if a plumb line can be lowered straight down next to the wall, its thickness must be measured exactly.

Shabbos 28:15

If the surveyors come to a mountain, then if the slope of the mountain is ten handbreadths (about 30") within a five cubits (about 7.5'), they measure the mountain and then resume normally. If the slope is steeper, rising ten handbreadths within four cubits (about 6'), then they approximate the mountain and resume. If the mountain is too wide for the 50-cubit rope to measure, i.e., the mountain is more than 50 cubits thick, then its thickness is approximated by measuring a little at a time. This is what the Mishna means when it says, "Mountains are measured by approximation" (Eiruvin 5:4).