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The Abney level is typically used at the eye height of the surveyor, either hand-held or mounted on a staff at that height. To measure lines on a particular slope, the desired angle or grade is first set on the level and then the surveyor sights through the sighting tube and brings the cross-hair in line with the bubble in the level while ...
Traditionally the instrument was completely adjusted manually to ensure a level line of sight, but modern automatic versions self-compensate for slight errors in the coarse levelling of the instrument, and are thereby quicker to use. The optical level should not be confused with a theodolite, which can also measure angles in the vertical plane.
A wooden tripod holding an optical level is set up firmly on the ground. Levelling or leveling (American English; see spelling differences) is a branch of surveying, the object of which is to establish or verify or measure the height of specified points relative to a datum.
Tree heights can be directly measured using a pole for shorter trees, or by climbing a larger tree and measuring the height via a long measuring tape. Pole measurements [30] [31] work well for small trees eliminating the need for trigonometry involving multiple triangles, and for trees shorter than the minimal range for laser rangefinders ...
A theodolite (/ θ i ˈ ɒ d ə ˌ l aɪ t /) [1] is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying , but it is also used extensively for building and infrastructure construction , and some specialized applications such ...
A Snellen chart is an eye chart that can be used to measure visual acuity.Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelius Donders.
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The visibility from an elevated observation point down to the surface of the sea can be calculated using the Pythagorean theorem, since the line of sight and the radius of the Earth form the two legs of a right triangle. The height of the elevated point plus the Earth radius form its hypotenuse. If both the eyes and the object are raised above ...