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To further a little what the others have posted, a "transit", or theodolite, will allow the scope to rotate both in a horizontal AND verticle plane. So, with a "transit", you "turn" verticle angles and "shoot" down or up a hill to a rod reading. Then, using trigonometry, you can calculate the elevation differential. You can also "shoot" trees and flag poles and calculate the height. Levels, as the others have said, cannot rotate in the verticle axis. But using the thumb screws to make sure the sight tube is level, you can use a process called "differential leveling" to get elevations of the ground, or building, by adding and subtacting the "rod readings". Its really handy to set level lines on foundations, form boards, or even trim work, also. Levels can either be manual, generally with four thumb leveling screws or semi automatic with two internal prisims hanging by wires inside the instrument that automatically levels it, with only three thumb screws. I'm not really sure of the term "optic" , as both a level and a theodolite are "optic". Unless they are talking about todays laser levels, that are also commomly used. Now a days distances are measured electronically, bouncing a laser off a prism pole to get horizontal didstnces. The old optic method, uses two stadia lines on the lense to get the horizontal distance. Submarines used the same principle to measure the distance to ships. Most stadia are set in the level or tranist to have the distance viewed between the top and bottom "hair" to read 1 foot at a 100 foot distance. So you can measure the distance to someone holding a rod by subtracting two numbers on the rod. Generally good to the nearest foot or so of horizontal distance. So with one "shot" you can get the elevation, horizontal angle from a "backsight", and the distance away, or what we call the x,y, abd z location, without using a tape measure.
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