The theory and practice of surveying. Designed for the use of surveyors and engineers generally, but especially for the use of students in engineering (1886)
Johnson, J. B. (John Butler), 1850-1902 · New York, J. Wiley & sons · leaf 569
s the actual difference ofelevation to be found. In precise spirit-levelling a special typeof the ordinary spirit or engineers level is used, and greatcare taken in the running of a line of levels from the sea-coastinland, connecting directly or indirectly with the triangulationstations and… … have to correct only for the curvature of a level surface at thelocality. It is found, however, that rays of light near the sur-face of the earth usually are curved downwards—that is, theirpaths are convex upwards. This curve is quite variable, some-times being actually convex downwards in some…
Surveying
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