USGS
Many of the early USGS datums were based upon assumed elevations that were obtained from various non-government sources. In order to begin the topographic mapping in a particular area, USGS chose a starting bench mark that was locally accepted even though the certainty of that bench mark being related to true sea level could not be ascertained. If all of the work done in a certain area was related to one starting bench mark, then in theory all bench marks could later be adjusted once the true elevation of the starting bench mark was known. In the Black Hills region of South Dakota the datum was known as the "Deadwood Datum". USGS began their work from a bench mark in the city of Deadwood that was established by the Fremont, Elkhorn & Missouri Valley Railroad (later the C&NW Railroad). This railroad had carried their elevations westward as best they could, but the accuracy was not to geodetic standards. A mark on the foundation ledge (water table) at the northwest corner of the Deadwood City Hall was the railroad's bench mark. USGS then set their own bronze tablet in the city hall building and established an elevation upon it based upon the elevation of the railroad's bench mark. This bronze tablet then became the starting point in which all bench marks throughout the Black Hills would be based and was known as "DW 4543". All Deadwood Datum bench marks have the "DW" designation and the number stamped upon them that was to the nearest foot in elevation.
The city hall building at Deadwood was consumed by fire in 1952 and has long been removed, but hundreds of bench marks in the Black Hills still remain that were referenced to this starting marker. Later when USC&GS came into the southern region of the Black Hills at Edgemont in 1908 while continuing their transcontinental leveling network, the elevations of the bench marks of the Deadwood Datum were adjusted. The Deadwood Datum was found to be approximately 1.40' lower than the USC&GS sea level datum.
As an example, the copper bolt bench mark "DW 4865", located at Mystic, South Dakota, has had the following elevations:
4865.325' - Original USGS elevation of the Deadwood Datum.
DEADWOOD DATUM
4866.725' - Adjusted elevation to USC&GS in approximately 1908.
4867.525' - Readjusted elevation to USC&GS in 1912.
4867.70' - Adjusted to NGVD 1929 Datum. (1934 Leveling).
4869.63' - Computed to NAVD 1988 Datum.
4869.66' - Adjusted to NAVD 1988 Datum. (1991 Leveling).
Text from USGS 19th Annual Report of 1897-98, page 281.
The city hall at Deadwood was once an impressive building, but burned in 1952 and is now a parking lot.
Photo is looking southeast.
The FE&MV Railroad depot in relationship to the old city hall location across the street to the west.
See Recovered Deadwood Datum
Bench Marks
(HERE)
© Jerry Penry 2014