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A major impediment to observing angles of large-area triangulation schemes was the curvature of the Earth. Assuming flat ground and an instrument height of 6 feet, the surveyor’s world was constrained within a circle with a radius of approximately 3 statute miles. This was unacceptable, and a variety of methods were devised to defeat the curvature of the earth. The first, and most obvious method, was to conduct observations from topographic highs. In flat country, this was supplemented by building towers of various sorts or observing from or to pre-existing structures. The Great Trigonometric Survey of India constructed many robust masonry towers for supporting surveyor and instruments, some of which had separation between an instrument stand and the observer’s platform. Some of these towers still exist today.
Roughly concurrent with the Indian Survey, the United States Coast Survey began running a chain of triangles down the East Coast of the United States. Both labor and materials were relatively more expensive in the United States than on the Indian sub-continent so masonry towers were out of the question. Edmund Blunt, in charge of carrying a triangulation scheme through the Delaware marshes in 1845, hit upon the idea of constructing wooden towers within towers to attain the desired intervisibility between stations of the primary and secondary triangulation schemes. The inner tower supported the instrument and the outer tower supported the observer. His highest towers were 45 feet high. Such towers were used sporadically in the years before the Civil War. During the war, the Army Signal Service built several towers that were over 100-feet high. This seems to have been the inspiration for the Coast Survey to use ever higher towers, particularly after it began the 39th Parallel Survey. One of the highest towers during this period was built at Station Green in Indiana, a 152-foot effort. However, this was dwarfed by the largest wooden tower ever constructed by the Coast and Geodetic Survey. This was built on Bugsuk Island in the southern Philippine Islands. It was a 235-foot behemoth constructed entirely of native timber cut on location. However, the days of the wooden towers were coming to an end as in 1927 Jasper Bilby and the Aeromotor Corporation developed portable steel survey towers that became known as Bilby towers. These graced the American landscape for over fifty years until the advent of GPS.
Albert "Skip" Theberge served as a NOAA Corps officer for 27 years prior to retirement in 1995. During that period he was primarily engaged in nautical charting and seafloor mapping but also served a stint in geodesy working on the Transcontinental Traverse project during the 1970s. For the past 18 years he has worked as a research librarian at the NOAA Central Library and has produced a number of historical works related to the Coast and Geodetic Survey (C&GS) and seafloor mapping. He also produced the NOAA History website (www.history.noaa.gov) and the NOAA Photo Library (www.photolib.noaa. gov) which includes thousands of historic photos related to the work of the C&GS.
A 5.344Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE