The Deadwood Datum

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When the U. S. Geological Survey (USGS) began their topographic mapping program in the 1880’s, one of the early problems they faced was the lack of precision horizontal and vertical monuments in the areas to be surveyed. Instead of progressing across the nation from areas with existing monuments to locations where new points needed to be established, USGS often had to begin mapping in isolated areas where no accurate geodetic surveys had taken place.

One such area was the Black Hills region of southwestern South Dakota and eastern Wyoming. The forestry and mining industry necessitated the mapping of that region before surveys initiated in other areas of the country could reach the Black Hills.

The horizontal triangulation network for the mapping of the Black Hills was established at Rapid City by placing an astronomical pier on the grounds of the Pennington County Courthouse in 1890. The brick pier was 5.5 feet long and extended three feet below the surface. The top was dressed with a sandstone cap and lettered U.S.G.S. A meridian point was established approximately one mile north of the pier.

It was Samuel S. Gannett, the younger brother of the famed Henry Gannett (Chief Geographer of the Geological Survey and founder of the topographic mapping program) who made the celestial observations. Samuel S. Gannett derived the pier’s latitude position by what was known as the "Talcott" or "Double Zenith Distance" method. Pairs of stars were selected from Safford’s Catalog of 2,018 Stars, or whenever possible from the Berliner Astronomisches Jahrbuch. The pier’s position for longitude was determined by the telegraphic method with the Washington Observatory at St. Louis, Missouri as the base station. The Fremont, Elkhorn & Missouri Valley Railroad had reached Rapid City in 1886 thus providing the means to obtain time via telegraph which was necessary for obtaining longitude.

As with any stand-alone triangulation network, there needed to be a measured base line. The Rapid City Base Line was located in the valley of Box Elder Creek, the most suitable location offering a prolonged area of flat terrain. The baseline was oriented in a mostly east-west direction with the west end just northeast of Rapid City and the east end just south of the present-day location of Ellsworth Air Force Base. The base line was 25,796.711 feet in length (4.886 miles), measured three different times with a 300-foot steel tape, the latest precision measuring device at that time. This early horizontal triangulation was the beginning of what was known as the Rapid City Astronomical Datum.

From Rapid City, a network of nineteen points expanded throughout the Black Hills. Triangulation stations, consisting of marked stones or copper bolts, were strategically placed on the tops of high peaks such as Harney, Custer, Terry, Bear Butte and others. In 1897, a meridian line was placed at Deadwood with nine additional triangulation stations and, in 1898, another meridian line at Custer.

Establishing the vertical network of precise bench marks posed a serious challenge for USGS because the precise levels established by the U. S. Coast & Geodetic Survey (USC&GS) along their transcontinental line was hundreds of miles away in central Kansas. Since there could be no further delay in starting the topographic mapping of the Black Hills, USGS chose a starting bench mark that was locally accepted even though the certainty of the elevation of that bench mark as related to true sea level could not be accurately ascertained. In theory, if all work done in one isolated area of the country was related to one starting bench mark, then all bench marks could later be adjusted once a connection was made with a bench mark related to sea-level. In the Black Hills region of South Dakota, the vertical datum established by USGS became known as the "Deadwood Datum".

The leveling for USGS began in 1897 from a bench mark located in the city of Deadwood which was earlier established by the Fremont, Elkhorn & Missouri Valley Railroad late in 1890. This railroad had carried elevations westward as best it could, but the accuracy was not to geodetic standards. A mark placed on the stone window ledge at the northwest corner of the Deadwood City Hall, just west of the railroad depot, was the railroad’s bench mark at 4544.73 feet. USGS then established their own bronze tablet in the stone lintel on the west side of the main door of the city hall building. USGS established an elevation upon it by transferring the elevation from the railroad’s bench mark. The elevation of the USGS bronze tablet bench mark was 4543.472 feet. This bronze tablet, known as "4543 DW", became the starting point from which all USGS bench marks throughout the Black Hills would be referenced. All Deadwood Datum bench marks are stamped with the "DW" designation followed by four numbers representing the nearest foot in elevation.

For primary lines, standard "Y" levels were used with the leveling done in closed circuits. The allowable closing error was 0.05 feet multiplied by the square root of the distance of the circuit in miles. Any error found meeting this tolerance was then distributed evenly throughout the benchmarks in the circuit. J. C. Barber, C. E. Worthington, J. T. Stewart, and L. F. Gottschalk did the field work in the Black Hills. Samuel S. Gannett was in charge of the computations, adjustments, and preparation of the bench mark lists. The permanent bench marks consisted of three main types. Flat, circular bronze or aluminum tablets, 3.5 inches in diameter and ¼ inch thick, with a 3-inch stem, were drilled and cemented into boulders, bridge abutments, or masonry walls on buildings. A one-inch diameter copper pin, referred to as a "bolt", was used primarily in rock outcroppings. The head of these bolts was stamped with the appropriate figures. The third type of bench mark was a 3.5-inch diameter wrought-iron post, 4 feet in length with a brass cap riveted to the top. The bottom of the post, flanged to prevent removal, was set three feet into the ground. Temporary bench marks included chiseled crosses, spikes in trees, or other identifiable locations. Most of the bench marks of the Deadwood Datum, along with their elevations to the nearest one thousandth of a foot, were published in 1901 in USGS Bulletin 185 "Results of Spirit Leveling Fiscal Year 1900-01".

During the next few years, the leveling was expanded south to the Nebraska border, west into Wyoming, and north to the area of Belle Fourche by using the same bench mark at Deadwood as the basis of the datum.

The USGS surveyors frequently used the railroads as routes to run their level lines. In the southwestern portion of the Black Hills, USGS utilized the Chicago, Burlington & Quincy Railroad for over 70 miles. Bench marks were established along this railroad through the towns of Ardmore, Edgemont, Marietta, and Newcastle, Wyoming. USC&GS later utilized this same railroad in 1908 for precise leveling. This agency established their own elevations upon 23 bench marks previously placed by USGS. Thus, a direct connection was made between the true sea-level based datum of USC&GS and the Deadwood Datum of USGS.

By comparing the elevations of common bench marks in the Edgemont area, it was discovered that the Deadwood Datum was about four feet lower than the elevations established by USC&GS on their sea-level datum. The difference in elevation of bench mark 3449 DW at Edgemont was 4.392 feet. Four other bench marks in the same general area ranged from 4.2
95 to 4.350 feet in elevation difference. Although this was quite a noticeable difference, it is remarkable that earlier elevations carried along the railroads from the east to Deadwood, half way across the country, with non-geodetic surveying methods would be that close to a true sea-level datum.

For unknown reasons, USGS adjusted the entire Deadwood Datum by adding exactly 1.400 feet instead of the more than four feet difference discovered by connecting with the USC&GS sea-level datum at Edgemont. These adjusted elevations were published in 1911 in USGS Bulletin 472 "Results of Spirit Leveling in South Dakota 1896 to 1910, Inclusive". If these adjusted elevations were intended to be directly relational to true sea-level datum, they were still approximately 2.992 feet too low.

The Fourth General Adjustment of the precise level net in the United States was made by USC&GS in 1912. The results of this adjustment, published in 1914 in USC&GS Special Publication No. 18, subtracted 2.075 feet from their elevation on bench mark 3449 DW at Edgemont. USGS accepted this new adjustment and then made their elevations on common bench marks to exactly match that of USC&GS. At this point, the difference at Edgemont was 2.317 feet from what USGS had originally established at bench mark 3449 DW and from what USC&GS determined it to be with the 1912 adjustment.

After the 1912 adjustment, the elevation of the USGS starting bench mark at Deadwood, 4543 DW, had changed by 2.200 feet from its original elevation. This number was likely the result of USGS taking more factors into consideration when performing a large regional vertical readjustment instead of changing each bench mark by the same value. As the USGS bench marks within the Deadwood Datum range from an elevation of approximately 2552 to 6219 feet, applying one simple shift would not have worked. The elevations resulting from the readjustment of the Deadwood Datum from the Fourth General Adjustment by USC&GS were published by USGS in Bulletin 643 "Results of Spirit Leveling in South Dakota 1896-1915, Inclusive".

In 1934, USC&GS established a new leveling line north from Edgemont and up through the center of the Black Hills along the Chicago, Burlington & Quincy Railroad. Approximately 25 bench marks, earlier established by USGS in the late 1890’s on the Deadwood Datum, were tied in by USC&GS along this route.

A large number of the bench marks established by USGS on the Deadwood Datum between 1897 and 1903 still exist today, especially in the remote areas of the Black Hills. The majority of these early bench marks, however, have never had precise levels established on them and the accuracy of the elevations remains uncertain.

The city hall at Deadwood was consumed by fire in 1952 and torn down. The starting bench mark of the Deadwood Datum, 4543 DW, placed on this building also was destroyed when the building was removed. Today, the site is a parking lot on the southeast corner of Deadwood and Main Streets.

The astronomical station at the courthouse in Rapid City was later affixed with a bronze tablet on the top during leveling in the city. This monument was likely destroyed when a new courthouse was built in 1922. The historical position of the monument is now in a parking lot. A few of the original triangulation stations survive in the Black Hills.

Author’s Note: The recoveries of the Deadwood Datum bench marks can be found at

Jerry Penry is a licensed land surveyor in Nebraska with nearly 30 years of experience. He is also a public speaker and a published author having written several historical books.

A 4.220Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

About the Author

Jerry Penry, PS

Jerry Penry has been surveying for 34 years, is licensed in Nebraska and South Dakota, and has been employed with Lancaster County Engineering for 21 years. He is also serving his second term on the Board for the Professional Surveyors Association of Nebraska.