Category: Surveying

The list below is believed complete, though membership – like everything here – is subject to revision.

Celestial navigation – One sight does not say where you are. It says you are somewhere on a circle three thousand miles across, and the craft is everything after that.
Deflection of the vertical – Down does not point at the centre of the Earth. The plumb line leans towards whatever rock is nearby, by a few seconds of arc, and the lean is a map.
Dip of the horizon – The angle by which the sea horizon falls below eye level, growing as the square root of the observer's height: in every almanac, and shaped like a radius.
Eight inches per mile, squared – The flat-Earth drop rule – a correct approximation of the Earth's curvature, cited chiefly by people who then fail to find it.
Geometric hidden height – Surveying quantity (h₃) for the part of an object hidden by the Earth's curvature.
Horizontal – The plane perpendicular to the plumb line – flat, and tangent to the level surface, but true only where it is taken; two distant horizontals are not parallel.
Level – The geodetic sense of the word: a surface everywhere perpendicular to gravity, and therefore – over any distance – curved. Still water is the demonstration.
The longitude problem – Not a question about where you are but about what time it is somewhere else. It was solved twice, by two methods that went to sea together for a century.
Hamish Lorimer – Scottish surveyor (1803–1878) who devised the star-occultation method, found it measured only refraction, and abandoned it for cause.