NAUTICAL SURVEYING Naval hydrographic surveying has since the termination of the war in 1918 steadily progressed in adopting the latest methods and instruments which have become available to the surveyor ashore and afloat. It is not often realized that the hydrographic surveyor has to combine the work of the land surveyor with his own.
Instruments.—The following are the principal instruments required for use in the field and it should be noted that various scientific inventions of great benefit to the surveyor, and of which full advantage has already been taken, were designed and per fected during the World War, 1914-18.
Theodolites (q.v.) in current use (1928) are 4 in., 5 in. and 6 in. The majority of these are micrometer theodolites reading to Io seconds. The use of the theodolite for astronomical and tache ometer work is now universal.
Sextants for observing with stand and artificial horizon are still supplied and improvements in this instrument have been adopted; amalgamated troughs consisting of gold-covered plates on which a thin film of mercury is floated have superseded the old artificial horizon consisting of a mercury bath ; the new pattern is far less sensitive to earth tremors. This instrument for work on shore is now to a great extent superseded by other more precise and com pact instruments.
Astrolabe a prisme, a very precise instrument, is one of these. It is used for finding position and enables altitudes of any stars at the altitude of 45° or 6o° to be observed. The latest form of astrolabe enables observations of stars to be easily and ac curately made as follows with the 6o° instrument—one step of 7' on either side of 6o°, that is 3 observations of 59° 52Y, 6o°, 6o° o71' can be taken of one star. With the 45° instrument— four steps of 5' on either side of 45°, that is II observations of 35', 45', 55', 05', 10', IS', 45° 20', 25'—can be taken of one star. The great advan tage of this instrument is that with one setting up of the instru ment and without a number of necessary readjustments, as in a theodolite, both time and latitude can be determined, provided of course that the best and latest method of obtaining error of the time used (i.e., wireless time signals) is adopted.
When it is found necessary to measure bases the hydrographic surveyor uses the 50o ft. steel measuring tape and is provided with the Kew standardization certificate. In surveying abroad where no local triangulation exists, the accurate measurement of a base is recognized as a most important step, second only to a satisfactory base extension. Tacheometers and tacheometer staves marked according to the Admiralty pattern are used for measuring dis tances up to over 2,000 f t. where extreme accuracy is not neces sary. One-metre base range-finders are useful in measuring short bases for plans of harbours, etc., when time or circumstances do not permit of a more accurate method.
Sounding Sextants differ from ordinary sextants in being lighter and handier; the arc which is of brass is cut to minutes, reading to large angles of as much as and fitted with a telescope of high power.
Station Pointer is in constant use for all kinds of plotting: it enables the observer's position to be fixed by two angles between three objects suitably placed, the centre of the instrument indi cating the observer's position when set and applied to the plotting sheet.
Sun Signals for reflecting the rays of the sun to distance stations for the purpose of obtaining accurate angles and measurements. The most convenient form is Galton's Sun Signal, which is easy to operate, compact and portable.
In addition pocket aneroid barometers for topographical pur poses, prismatic compasses, patent logs, Lucas sounding machines, both large and small, James's submarine sentry, tacheometer staves, etc., are also required. For chart room use graduated brass scales, steel straight edges, beam compasses of various lengths, rectangular protractors, circular brass protractors, mathematical drawing instruments, weights, drawing boards and paper are required.