This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1918 Excerpt: ... sea-level and the height of one point in the triangulation above sea-level is established. By using the vertical arc of the theodolite the difference in height between the various trigonometrical stations is obtained concurrently with the measurement of the horizontal angles and the altitudes of the stations are thus permanently fixed. To discover the position of the island on the Earth astronomical observations are made to fix the latitude and longitude of one point in the triangulation. A second astronomical observation is necessary for ' Azimuth ', i. e. to obtain the true bearing of the base or one side of the triangulation, and therefore the relation of the whole triangulation to True North. Before the results of the survey can be plotted on paper ready for the subsequent detail work a ' Projection ' must be decided upon, projection being the term given to the method of drawing the meridians and parallels of the earth on a flat plane. The positions of all the trigonometrical stations are then computed, and, if the triangulation has been properly carried out, the sum of the observed angles in any triangle should not differ from 180 by more than 5". From the computation the co-ordinates, or distances N. S. E, and W. of the lines of origin, are obtained for all the trigonometrical stations, and these are then plotted on the plan. To carry out the detail surveying, that is to say the mapping of all the topographical details and representation of the land-relief by contours, the island is divided up into sections, and sheets, each containing several of the trigonometrical stations, are divided up amongst the detail surveyors or ' Plane-tablers'. By means of the trigonometrical stations plotted on his sheet and visible on the ground the plane-tabler ca...
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