Image from page 190 of "Maps and survey" (1913)
Title: Maps and survey
Publisher: Cambridge, University press
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress
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Reconnaissance ladder incourse of erection. 3. Double tripod scaffold fortheodolite and observer. fieconnaissance ladder and beacon.Service Geographique de VArniee. Designed by Commandant L. Ditrand,11 Rcpinient dAriillerie. TOPOGRAPHICAL SURVEY 137 a chain should start from one base and close on another. Thenthe comparison between the length of the second calculatedthrough from the first and the length of the second as actuallymeasured gives the best possible control over the accuracy ofthe whole chain.
Text Appearing After Image:
Fig. 18. Diagram of Geodetic Triangulation up the Nile Valley from Cairoto Beba. The two heavy lines are bases. Beacons. Beacons are of two principal kinds : the luminous and theopaque. In most countries, when the ray is more than eight ornine miles long it is necessary to use luminous beacons or 138 TOPOGRAPHICAL SURVEY signals, for the haze in the air very quickly obliterates thecontrast between the opaque signal and its surroundings, so thatit becomes invisible. Luminous signals are either heliographs, for use with the Sunby day; or powerful lamps, nowadays usually acetylene, forwork at night. In a fine climate like South Africa, the heliocan be observed at a range of lOO miles and sunlight is sufficientto let the work proceed without undue delay. In less favourableclimates lamps provide a more certain mark for observation ;but there may be difficulties in observing at night, for reasons ofhealth or safety, that make it necessary to restrict operationsto the day. On cloudy days it
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