Maps and survey . d to sea level also. The details of the manipulation of the theodolite are dealtwith in the Textbook of Topographical Surveying. We willconfine ourselves here to the points which are especially con-cerned with the horizontal triangulation. Suppose that there are four stations A, B, C, and D to beobserved. A round of angles consists of settings on A, B, C, D,and A in turn. It is important to notice that the station A isobserved to twice. The reason for this is that small errors areintroduced into the measured angles if the instrument is notperfectly stable during the course of
Maps and survey . d to sea level also. The details of the manipulation of the theodolite are dealtwith in the Textbook of Topographical Surveying. We willconfine ourselves here to the points which are especially con-cerned with the horizontal triangulation. Suppose that there are four stations A, B, C, and D to beobserved. A round of angles consists of settings on A, B, C, D,and A in turn. It is important to notice that the station A isobserved to twice. The reason for this is that small errors areintroduced into the measured angles if the instrument is notperfectly stable during the course of the round ; and this cannotbe ensured absolutely. If we repeat the observation o{ A at theend of the round we ensure that the measure of the angle DO Ais independent of any settlement of the instrument that may ISO TOPOGRAPHICAL SURVEY have occurred during the course of the round from A to D. Infact any such movement affects only the particular angle undermeasurement at the time when it occurred. At the same time. Fig. 19. the near concordance that there should be between the first andthe last settings on ^ is a measure of the stability of the instru-ment during the round. This is an important principle, to whichattention should be paid. There are, however, cases in which the simple processdescribed above must be modified. In a bad climate it willhappen that sometimes one and sometimes another station isvisible ; but never all at once. In such a case one establishes areference mark at such a distance that it is always visible; andone measures the angle between this mark and any other stationthat may become visible. The differences between the bearingsof all the stations from this reference mark give eventually theangles that should have been observed directly. For simplicitywe shall assume in what follows that the whole round of anglescan be observed in the manner first described. To obtain the necessary control and to detect blunders, aswell as to eliminate by repetition the
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectsurveying, bookyear19
