According to surveyors James R. and Roy H. Wirshing, the difference between a theodolite and a transit is a matter of debate, with no clear consensus; some instruments are referred to as transit/theodolites. There is certainly a good deal of overlap between the functionality of these two surveying tools, although the theodolite, especially the digital theodolite, is nowadays used more widely.
The transit was developed in the U.S. in the 19th century during the expansion of the railways. They are useful for viewing long straight distances via their telescopes. Both thedolites and transits have telescopes; according to geomatics engineers Charles Ghilani and Paul Wolf, theodolites are characterized by having short telescopes. The Wirshings state that "transiting" means to reverse or invert a telescope, which is how the transit works and how it gets its name.
Accurate measuring is vital for surveying and meteorology, two of the main applications of theodolites and transits. A vernier scale is an extra sliding scale that allows for additional precision by the “fine tuning” of measurements. Transits usually have metal circles that are read by means of verniers, whereas theodolites have glass circles and micrometers, devices that incorporate a calibrated screw. In today's theodolites, the readouts from the horizontal and vertical circles are provided electronically.
Ghilani and Wolf claim that a key feature of a theodolite is its collimator -- a device that narrows a beam of light. A collimator allows quick pointing, or fast and accurate positioning of the optical axis. Collimators give theodolites extra precision when measuring angles, according to surveying technician Paul Kunkel. The additional speed and accuracy of a theodolite over a transit, in many situations, is one reason why it has become more popular.
Modern theodolites have laser beam-emission capability. This gives them the capacity to work over much larger distances than older theodolites and transits. One model, built by a Japanese company, boasts a laser range of nearly 2000 feet. Further, whereas transits tend to have specific uses, more modern theodolites can be used for a range of purposes. These include surveying during tunnel excavations, supplying information to allow precise alignment of modules during construction and a wide range of engineering jobs.