Nowadays we use lazers to give us lines and levels.
A theodolite is a precision instrument for measuring angles
in the horizontal and vertical planes. I have never used this particular one but i could if i had the correct tripod mounting.
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Bubble to set level of instrument
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Bubble to set level of telescope
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This fitting is either screw on or is designed to fit on a TRIG point or triangulation station for mapping purposes
https://en.wikipedia.org/wiki/Triangulation_station
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the view through the telescope, the view is upside down so the measuring staff (or big ruler) would be held upside down to make it easier to make the measurement.
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The surveyors assistant or "chainboy" would lay out a chain (exactly 22 yards) to mark the distance and the engineer/surveyor would use the theodolite to measure angle and height differences.
Early mapmakers used devices similar to this, i have no idea how old this one is, my guess is 1920,s, i know it was last calibrated 50 years ago.