A Theodolite is a instrument for measuring both horizontal and vertical angles, as used in triangulation networks, and geo-location work. It is a tool used in the land surveying and engineering industry, but theodolites have been adapted for other specialized purposes as well. Other specialized purposes make Theodolites ideal for shop and factory floor layout of tools and fixtures. They also work well for layout for the construction of concrete slabs, swimming pools, golf courses, landscaping, and road design.
The horizontal accuracy of Theodolites depends on "seconds". A 2-second theodolite is more accurate than a 5 or 9-second theodolite. If you think about the horizontal circle that a theodolite rotates around, the circle is divided into 360 degrees. Each degree is divided into 60 minutes, and each minute divided into 60 seconds. Think "Degrees / Minutes / Seconds". The horizontal angle is the measure of inaccuracy (hence accuracy) that a theodolite can horizontally measure or locate within. If a theodolites accuracy rating is 2 seconds (written 2") then its only going to lose 2 seconds of horizontal measurement in a given distance. Generally speaking, a 9 second theodolite is for construction sites where you're working relatively up close, say within 200 feet from the instrument. A 2 second you would work 2,000 feet away and still work with some level of accruacy. Most building contractors, whether residential or commercial, can use a 9 second theodolite without experiencing problems due to accuracy. At this distance, more errors are in the form of human errors, such as not leveling the instrument properly or taking a quick reading which lends itself to human error.