Measure the techum Shabbat with a rope that was 50 amot long, precisely, as held at the surveyor's heart. And if there's terrain "in the way," the surveyor holds the rope over the area that is not flat as if it's flat... as long as they don't leave the techum to do so, lest their trek mislead spectators. Alternatively, they could "pierce" the hills to draw a flat line, as it were. Note that the rope should be made from a material that won't give it shrink in the weather or after usage - as long as it is actually rope. Note also that for the cases of surveying for the eglah arufah or for the cities of refuge could not be done through the piercing method, because the measurements have to be accurate, as per the Torah.