In simple terms, a datum is something used as a basis for calculating or measuring in which all things are referred to a common base system. In surveying, a datum is a reference system, both horizontal and vertical, to measure and compare positions on the surface or heights above and below that surface. To be accurate, one should always state the associated datum after the given elevation.
In regard to elevations, such as the one associated with Harney Peak, one must first know what datum that elevation is associated with in order to accurately relate it to future measurements. The knowledge of the earth is always changing and with increased knowledge there have been differing datums. Vertical datums are generally associated with the height above the average level of the sea which has a base elevation of 0 (zero). As elevations were carried across the United States, they were related to that base elevation. Naturally, there were cumulative errors, both human and mathematical, too complicated to address here.
When the U. S. Geological Survey (USGS) began surveying in the Black Hills in the 1890's, a vertical datum was needed to relate all places within the Black Hills together so that elevations could be relational to each other. An elevation supplied to them by the FE&MV Railroad in Deadwood was chosen as that base. This elevation was based upon sea level, but because there could be error in this elevation it needed to be confined to just the Black Hills. This datum became known as the "Deadwood Datum".
Undoubtedly, the first accurate elevation of Harney Peak in 1898, 7240', was related to the Deadwood Datum. However, this is never stated. In 1908, the U. S. Coast & Geodetic Survey (USC&GS) passed through the southern Black Hills and tied into points that had elevations already associated with the Deadwood Datum. This sea-level based datum carried along by USC&GS was found to be around 1.40' higher than the Deadwood Datum. USGS adopted the USC&GS datum and mathematically adjusted the elevations on all their points. Nothing was physically moved, the published elevations just changed on paper.
In 1912, with increased data, USC&GS adjusted their datum in what was known as the Fourth General Adjustment. This increased the elevations of points within the Black Hills by another 0.80'. Again, a shift was made on paper by rewriting the stated elevations for each point. In 1929, more adjustments were made and the National Geodetic Vertical Datum was created (NGVD29). This increased the elevations by another 0.18'. Then in 1988, a new datum known as the North American Vertical Datum (NAVD88) was created by the National Geodetic Survey (NGS) which adjusted the elevations higher another 1.96'. The next new datum will be in 2022 which will lower the elevations of 1988 by around 2.4' in the Black Hills. This is known as the National Spatial Reference System (NSRS).
To put this in perspective, the elevation of the highest remaining rock on Harney Peak today is 7231.32' NAVD88. This list will give you the elevations of the same location for the past, present, and projected future as new datums were created:
7226.99' - USGS Deadwood Datum 1898
7228.39' - USC&GS Sea Level Datum 1908
7229.19' - USC&GS Adjusted Datum of 1912
7229.36' - USC&GS NAVD29
7231.32' - NGS NAVD88
7228.91' - NSRS 2022 (Projected)
Rounded to the nearest whole foot, the current high point of Harney Peak today is 7231'. In 2022, it will change to 7229'.
DEADWOOD DATUM ARTICLE
© Jerry Penry 2016