EARLY VERTICAL DATUMS
BENCH MARK PHOTOS CODE LIST FINDING BENCH MARKS
During the season of 1896, the U. S. Geological Survey field parties engaged in the topographic mapping of the United States conducted careful spirit leveling on a scale more extended and of a quality more accurate than had ever before been attempted. This work was in pursuance of the Act of Congress for the fiscal year 1896-97, a paragraph of which read as follows:
"For topographic surveys in various portions of the United States, provided, that hereafter in such surveys west of the ninety-fifth meridian, elevations above a base level located in each area under survey shall be determined and marked on the ground by iron or stone posts or permanent bench marks, at least two such posts or bench marks to be established in each township or equivalent area, except in the forest-clad and mountain areas, where at least one shall be established, and these shall be placed, whenever practicable, near the township corners of the public-land surveys; and in the areas east of the ninety-fifth meridian at least one such post or bench mark shall be similarly established in each area equivalent to the area of a township of the public-land surveys.
In the beginning, it was realized by the framers of the above act that it would be impossible to accurately establish and correctly mark on the monuments their exact height above sea level. Any attempt to do this would have necessitated the running of thousands of miles of precise levels in order to connect the initial points within the various areas under topographic survey with sea level. This would have cost immense sums of money and occupied several years, during which the topographic surveys would have continued without being accompanied by spirit leveling and the establishment of the bench marks required. The act was therefore framed so as to permit the acceptance of some point within each area under topographic survey as a central datum point for that area, and the elevation of the initial bench mark established there was determined as nearly as practicable from existing elevations adjusted by railway levels brought from the sea.    (The above three paragraphs are taken from the U. S. Geological Survey Eighteenth Annual Report, Part 1.)
The early datums were utilized between 1896 and approximately 1915. Some areas of the country continued to use the early datums, but with more widespread precise levels by USC&GS, individual and isolated datums became rare. By the 1930's, USGS switched to using domed bronze and aluminum disks that were placed in concrete posts. For the most part, USGS then almost exclusively placed their bench marks on the North American sea-level datum of 1929. Many of the early isolated datums, however, were never releveled. The early bench marks placed on these individual datums still retain the original elevations based upon those individual datums. Many bulletins were published by USGS that listed these datums, the bench mark locations, and their elevations by state.
The early permanent bench marks established by USGS consisted of three standard styles.
1.   A flat circular bronze or aluminum tablet, 3½ inches in diameter and ¼-inch thick with a 3-inch stem to be cemented into a drill hole in the walls of public buildings, bridge abutments, or in solid rock.
2.   A copper bolt one-inch in diameter and 4 inches in length to also be placed in masonry or rock structures. A brass wedge at the bottom of the hole would expand the slotted end of the copper bolt when it was driven down into the hole upon the wedge.
3.   A hollow wrought-iron post, 4-feet 6-inches in length and 3½ inches in diameter with a split bottom. On the top of the post is a riveted bronze cap.
A fourth type was a copper nail/washer used for intermediate or temporary bench marks between the permanent types listed above. The washer was stamped U.S.G.S. B.M. and was ⅞-inch diameter with a 1 ¼-inch long copper nail.
Bronze Tablet, Copper Bolt, Iron Post, and Copper Washer.
This drawing was in nearly every USGS bulletin that listed bench marks by individual states.
© Jerry Penry 2013