During the 1896 field season, the surveying crews of the U. S. Geological Survey (USGS) engaged in higher quality and more precise topographic mapping and spirit leveling than had been previously attempted. This change was largely prompted by an act issued by the Fifty-Fourth Congress on June 11, 1896. For USGS, the most significant result of the act was that permanent bench marks on an approximate sea-level based datum were to be placed in each area of the new surveys. These bench marks were to be marked on the ground with at least two monuments established per township, except in forested and mountainous areas where at least one per township was required. Although USGS had already performed topographic mapping in many regions of the country for 30-minute quad sheets, permanent bench marks had not been placed in those areas.
The framers of the act knew it would be impossible for USGS to accurately record the monuments to their exact height above sea level. Attempting to do so would have required running thousands of miles of precise levels from points already known to be accurate. This would not have been financially feasible and it would have taken years to accomplish, greatly delaying the mapping program.
Therefore, a plan was designed to permit the acceptance of one fixed monument within each region of a particular topographic mapping area to be used as a central datum only for that area. The elevation of each initial bench mark, regardless of its relationship to the true sea-level datum, was usually derived from an outside source such as a railroad company, a river commission, or a local city datum. The elevations of these starting bench marks were of uncertain precision, but the assumption was made that they were very close to sea-level datum. All subsequent bench marks placed from the initial bench mark were then directly related to that monument, but they could not be interchanged with bench marks established from other datums. It was anticipated that through the course of the work, connections would eventually be made with precise monuments established by an agency such as the U. S. Coast & Geodetic Survey (USC&GS). Thus, adjustments in elevation could later be made to the USGS monuments without having to delay the mapping program.
Initially, the plan did not implement a specific code or numbering system for the bench marks. Only the approximate elevation was hand-stamped on the monument to the nearest foot. This stamped number individually identified each bench mark within the datum except in cases where two or more bench marks within the same datum had the same elevation. This system quickly proved insufficient since datums originating from different sources began overlapping into common areas which would make it impossible for surveyors and engineers to determine from which datum each bench mark originated.
In an improved system during the second year, each datum was assigned a specific code name which was stamped onto the bench mark in addition to the elevation. In theory, if a surveyor leveling between any two bench marks discovered that they did not close, the differing code names on the monuments was supposed to prompt the surveyor to contact USGS to inquire as to the problem.
The exact number of early USGS vertical datums is unknown since a concise list has not been found, but published records indicate there were hundreds of different datums located across the United States. The coding for the datums was apparently left to the individual party chief of each crew without much communication between crews working in other parts of the country. The same code names, therefore, existed in different states which were based upon entirely different datums. For instance, the letter "A" was a code used for different datums originating near Anniston, AL; Alexandria, MO; Asheville, NC; Albany, NY; Athens, OH; and Astoria, OR. The letter "B" was a code used for different datums near Benicia, CA; Bannock County, ID; Bangor, ME; Brockport, NY; Blaine, WA; and Baraboo, WI. The "LA" code was used for datums originating at Lafayette, LA, and Los Angeles, CA. Some datums were designed by a date instead of a letter code. The "1906" datum in Colorado had a different origin than the "1906" datums established in Illinois, Kentucky and Oregon. Most codes, however, had some indication of the city name where the datum originated such as "CHYN" for Cheyenne, WY; "GAINV" for Gainesville, TX; "MSLA" for Missoula, MT; "NOGLS" for Nogales, AZ; "VAN HN" for Van Horn, TX; or "YNKTN" for Yankton, SD.
There were variations to the codes within the same datum such as "D", "DENV", and "DENVER" in Colorado; "MIL" and "MILWAUKEE" in Wisconsin; "MLT" and "MALTA" in Montana; and "WP", "WILLETS", and "WILLETS POINT" in New York. These variations were likely the result of individual field crews abbreviating the codes within the same datum and not being consistent with the stamping.
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