779.—Stations of Observation vary materially according to the extent of the survey and its purpose. For geodetic works stations are raised at great expense, often in masonry or solid woodwork. For ordinary local or civil surveys the stations are commonly formed of single poles set up vertically, which vary in dimensions according to the extent of survey and the difficulties which may be encountered by various obstructions to direct visions by woods, lakes, marshes, etc. The apparatus that may be useful in the work of the civil engineer in ordinary practice will only be considered here.
Fig. 383.—Ranging pole or picket.
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780.—Pickets or Ranging Poles, Fig. 383, as the name indicates, are used for ranging a direct line through a district, either by a series of poles sighted from one to the other or by being placed in position convenient for triangulating by the theodolite where the country is open, or free from many buildings, trees, or other convenient landmarks.
781.—The picket (Fr. piquet) is a straight, slightly tapering pole shod with wrought iron or steel. It is generally made of about 1-1/8 inches diameter, and is painted in alternate feet red and white with an enamel paint that will not soil the hands or take dirt from them. The shoes should be made with strap-pieces, so that the picket, which is generally made of yellow pine for lightness, should not be liable to break off at the shoe in use. Fig. 381 represents the lower part of a picket as made by the author: B black, W white, R red. It is usual to have six pickets at least out in use with a theodolite in open country.
Fig. 384.—False picket.
Fig. 385.—Spur-shod picket.
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