The practice of plane surveying (determining the position of buildings, boundaries, and topographical features as if they were located on a flat surface) has a long history, dating back to the ancient Egyptians. In the United States, a couple of early American all-stars, George Washington and Thomas Jefferson, were both trained surveyors.
The transit level is to plane surveying what the hammer is to nailing. It's an optical instrument (essentially a telescope) mounted on a tripod along with a built-in spirit level. In combination with tape rules and calibrated rods, the transit compass, as it is also known, allows its user to determine the relative position of points, lines, and objects. Such determinations are useful in executing maps or plans and, in some cases, construction.
A transit level is a precision instrument. It is calibrated to indicate not only true horizontal but also to provide a reading of the angle of inclination in degrees, minutes, and seconds. In the hands of a professional, a transit level is a varied and flexible instrument capable of many tasks, but some of its most basic applications are easy to master. As a result, this tool can be invaluable to anyone planning to build a new structure.
The tool works like this. The transit itself is leveled using the spirit level. The telescope then can be pivoted on a horizontal axis to point in any direction. With the help of an assistant who positions the graduated rod, the transit operator can then sight the transit on the rod and determine the relative height of the grade or the object on which the rod is located.
The transit level, thus, is invaluable for excavation and foundation work, as well as for landscaping. Some framing contractors, especially timber framers, also find it very useful when sighting in on flooring or ceiling surfaces to ascertain level.
There are remarkable antique transits to be found in collections and shops. I've seen some amazingly sophisticated new models in the hands of professionals that feature a variety of bells and whistles few homeowners could ever make use of, including computer chips and laser beams that extend the reading of the level to one hundred feet or more. But there are also some very practical new models to be bought for about $200.
A basic setup should consist of the transit level itself, a tripod base, a calibrated rod, and a windup tape measure, perhaps one hundred feet or more in length.