It sometimes happens that both intersections of the bridge tangent with the base-lines cannot be seen from one end of one of the latter. In this case it will be necessary to put in a hub on the bridge tangent far enough ahead of the hidden point to clear the obstruction, triangulate to it, and measure the exact distance from it to the hub on the base-line. This expedient was necessary in the triangulation for the author's Jefferson City highway bridge.

A check on the accuracy of the triangulation work is obtained by comparing the two computed lengths of the bridge tangent between the intersections thereof with the base lines, or between one such intersection and a fixed point on the tangent on the other side of the river. The disagreement in these two measurements should be within the limit of one half of an inch to the one thousand feet. To show how accurately such work can be done, the author would state that for the Jefferson City bridge he gave his resident engineer instructions to allow no variation from correctness exceeding three eighths of an inch in either the main triangulation itself or in the intersections for pier centres. His instructions were followed so faithfully that no error exceeding three sixteenths of an inch was allowed to pass in any part of the work. The whole field-force once lost an entire half-day in rectifying an error of one half of an inch in the intersections for a pier centre. This is an excellent record for accuracy, considering that the distance between base-lines on the bridge tangent was a little over fifteen hundred feet. The author is generally not so rigid in his requirements for exactness as he was in this case, the reason for such strict instructions being the fact that this was the resident engineer's first experience in important triangulation.

The triangulation for the author's Sioux City bridge, made by Lee Treadwell, Mem. Am. Soc. C. E., with a bridge tangent about twenty-two hundred feet long between base-lines, was probably just as accurate as that for the Jefferson City bridge, because the errors in distances between pier-centres measured on top of the falsework were actually inappreciable.