The underground marks were set to preserve the surface mark’s position if the surface mark was damaged or destroyed. First the lower mark was set in a small mass on concrete, then a layer of dirt was added to isolate the lower mark, then the concrete monument was added, and then the surface mark was set directly over the underground mark. In addition, an underground mark may have been set about four feet beneath the surface. The main station, marked with a Triangulation Station disk, contains the factory stamping “TRIANGULATION STATION” with an equilateral triangle in the center. Note, other agencies may have used the same station name so care must be taken in identifying the correct agency’s survey disk. However, there may be 6 or more survey disks in the vicinity, counting underground mark(s), additional reference mark(s), and mark(s) of other organizations. The standard for many years was to set four disks at ground surface level in the vicinity of a Triangulation Station. Later, if a leveling survey crew was nearby, they may have leveled to one or more of the disks providing more accurate elevations. The triangulation method thus produces accurate horizontal positions but only approximate elevations. The goal of this procedure is to determine the horizontal positions (latitude and longitude) of the vertices of each triangle (the points marked by TRIANGULATION STATION disks). Triangulation consists of observing the angles at the vertices of adjacent triangles, measuring the lengths of some sides, and computing the lengths of the remaining sides. TRIANGULATION STATION - A Triangulation Station is a survey point established during a survey utilizing the triangulation surveying method. Paragraphs are from a paper I wrote some time ago explaining some of the different USC&GS survey disks. Underground marks may also have been set, see details below. Standard procedure for the USC&GS (and after the name change in 1970, the National Geodetic Survey) was to set four disks at ground surface level, a TRIANGULATION STATION disk (where the survey observations were made), two REFERENCE MARK disks, and one AZIMUTH MARK disk. Coast & Geodetic Survey (USC&GS) Triangulation Station. Recently there have been several comments and questions about the different disks in the immediate vicinity of a U.S. COAST & GEODETIC SURVEY TRIANGULATION STATION
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