881. Measuring Ditches and Valleys
The previous halacha (about measuring the width of a ditch that is up to 4,000 cubits deep) applies when a plumb line (which measures verticals) can be lowered straight down because in such a case the walls of the ditch cannot be used. However, if a plumb line cannot be lowered straight down, the surveyors should only measure the width of the ditch using the rope when the ditch is 2,000 cubits deep or less.
If a valley has a gradual slope, the surveyors should go down and up, measuring its approximate width. If the valley is more than 50 cubits wide - and therefore unable to be measured using the 50-cubit rope, the surveyors should go somewhere the gap can be spanned, measure it there, then return parallel from that spot to where they were measuring and resume.