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The necessity for extreme accuracy in the triangulation for piers of long bridges is not generally recognized; hence result errors in pier location that sometimes require the lengthening or shortening of the superstructure, or which involve the adoption of an unanticipated skew. There is no excuse whatsoever for any such errors in location, because the method of triangulation adopted should provide a check against not only blunders, but also even trifling variations from correctness of position, and because the Contractor should invariably, at the outset of his work, take such precautions as will prevent the occurrence of any variation in sinking in excess of that provided for in the Engineer's plans.
In the triangulations for bridges over large rivers, such as the Missouri, the author makes a practice of measuring each base-line five times and each angle thirty times; and no point is ever located without using a check from another base-line, thus providing an intersection of three lines, which theoretically should be a mathematical point, but which actually varies therefrom, generally about a quarter of an inch, and sometimes even as much as one half of an inch, in sights of about one thousand feet length.
The author has tried both iron rods and steel tapes for measuring base-lines, and has adopted the latter as the more accurate. The objection to using rods is that it is almost impossible to run a line a thousand feet long with three rods that must always be made to actually touch each other without sometimes disturbing slightly the position of two of the rods, when either lifting or putting down the third rod. With
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