[U.S. Coast & Geodetic Survey Disks]

DISKS IN THE VICINITY OF A U.S. COAST & GEODETIC SURVEY 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 (USC&GS) Triangulation Station. 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. Underground marks may also have been set, see details below. The following

paragraphs are from a paper I wrote some time ago explaining some of the different USC&GS survey disks.

 

TRIANGULATION STATION - A Triangulation Station is a survey point established during a survey utilizing the triangulation surveying method. 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. 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). The triangulation method thus produces accurate horizontal positions but only approximate elevations. Later, if a leveling survey crew was nearby, they may have leveled to one or more of the disks providing more accurate elevations.

 

The standard for many years was to set four disks at ground surface level in the vicinity of a Triangulation Station. 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. Note, other agencies may have used the same station name so care must be taken in identifying the correct agency’s survey disk. The main station, marked with a Triangulation Station disk, contains the factory stamping “TRIANGULATION STATION” with an equilateral triangle in the center. In addition, an underground mark may have been set about four feet beneath the surface. 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. The underground marks were set to preserve the surface mark’s position if the surface mark was damaged or destroyed. Both disks contained the same factory stamping (“TRIANGULATION STATION”) and contain the exact same stamped designation (name) and date. Triangulation Stations were normally named for an area feature or for the property owner. Just prior to setting, the disk would be stamped by the original surveyor, for example, “JONES 1936”. In the NGS database, the name would be “JONES” and the year set 1936. The disk was usually set so that the stamping could be read by an observer facing north. The surface disk would be set in a concrete monument buried in the ground, or set in a drill-hole in a large structure or bedrock. The concrete monument is normally about flush with the ground’s surface, 12 inches in diameter, and 48 inches or more deep, with the bottom larger in diameter to help resist frost heave. The USC&GS TRIANGULATION STATION type of disks were used from about 1900 to about 1970 (although there were several different versions). After about 1970, National Geodetic Survey HORIZONTAL CONTROL MARK disks were used.

 

REFERENCE MARKS - Reference Marks (RM) were set to assist in locating the Triangulation Station and also to help determine if the Triangulation Station was undisturbed in its original position. The measured directions and distances to them could also be used to reset a station mark if required. Reference Marks were factory stamped with “REFERENCE MARK” and with an arrow used to point in the direction to the Triangulation station. The original surveyor stamped the RM with the name of the Triangulation Station plus the number of the RM, just prior to setting. For example, the first RM for station JONES would be stamped “JONES NO. 1 1936”. The surveyor measured the direction and distance from the triangulation station to the Reference Mark (RM) and recorded the information as part of the station’s description. Later, if a surveyor attempting to find a Triangulation Station stumbled upon a RM first, the arrow and the published distance and direction between the RM and station would be valuable aids in the station recovery. To check the position of the Triangulation Station, the new surveyor could measure the angles and distances to the Reference Marks and compare them to the original values. USC&GS specified a Reference Mark as early as 1913. By the 1920’s, two Reference Marks per Triangulation Station were specified. Reference Marks were usually set within 30 meters (one tape length) of the station. Reference marks were numbered clockwise from north and set about 90 degrees apart. If a RM was destroyed, a new Reference Mark would be set using the next consecutive number. The disks would be set in a concrete monument buried in the ground, or set in a drill-hole in a large structure or bedrock. This type of disk was used from about 1913 to about 1970. Although the distance and direction provided enough information to compute the positions of the RMs, it was not standard procedure to compute them. After about 1970, National Geodetic Survey REFERENCE MARK disks were used.

 

AZIMUTH MARKS – Beginning in 1927, a third Reference Mark, or long RM, was set about ¼ mile away from a station. They provided a starting azimuth (direction) for local surveys and for determining magnetic declination (difference between true north and magnetic north). Standard Azimuth Mark disks replaced azimuth reference marks about 1935. Also in 1935 the precision of the directions to Azimuth Marks was increased by changing the number of repetitions of the angle measurements from two to four. Azimuth Marks visible from the ground at the main triangulation station had been frequently requested by local surveyors and engineers. Azimuth Marks were factory stamped with “AZIMUTH MARK” and with an arrow. The original surveyor stamped the Triangulation Station’s name and date on the Azimuth Mark disk, just prior to setting. Since this is the exact same stamping as on the Triangulation Station disk, persons recovering the mark must check the factory stamping of “AZIMUTH MARK” (with arrow) versus “TRIANGULATION STATION” (with triangle) to determine which is which. When set, the surveyor rotated the Azimuth Mark disk until its arrow pointed directly toward the Triangulation Station disk. The surveyor then measured the direction, and in later years distance, to the Azimuth Mark (from the Triangulation Station) and recorded the information. Measuring the distance to the Azimuth Mark began in the mid 1970’s when electronic distance measuring equipment came into common usage, and underground marks were set at many of these. Azimuth Marks were usually set between ¼ mile and 2 miles from the Triangulation Station, at a location that was visible from tripod height at the Triangulation Station, and generally in or near a fence line along a road. The Azimuth Mark was included in the “To Reach” portion of the station’s description. The distance to most Azimuth Marks was measured with an odometer, so most don’t have adjusted positions and many are a challenge to recover. The disk would be set in a concrete monument buried in the ground, or set in a drill-hole in a large structure or bedrock. This type of disk was used from about 1935 to about 1970. After about 1970, National Geodetic Survey AZIMUTH MARK disks were used.

 

Are the Benchmarks referenced to a master Benchmark or position?

 

thanks, Sam