Transit telescopes are equipped with three horizontal cross hairs which are mounted on the cross-hair ring. The top and bottom hairs are called stadia hairs. The surveyor sights through the telescope and takes readings where the stadia hairs intersect a scaled rod. The difference between the two readings is called the rod interval. […] The stadia method has its greatest value in locating details for maps, but it is also usefulf for making rough surveys and for checking more precise ones.