Publisher's Synopsis
Excerpt from Results of Spirit Leveling in Minnesota: 1897 to 1910, Inclusive
Bench marks - The standard bench marks are Of two forms. The first form is a circular bronze or aluminum tablet (0 and E, Pl. I), 3} inches in diameter and one-fourth inch thick, having a 3 - inch stem, which is cemented in a drill hole in solid rock in the wall of some public building, a bridge abutment, or other substantial masonry structure. The second form (f, Pl. I), used where masonry or rock is not available, consists of a hollow wrought - iron post 3) inches in outer diameter and 4 feet in length after being split at the bottom and expanded to a width of 10 inches in order to give a firm bearing on the earth. A bronze or aluminum-bronze cap is riveted over the top Of the post, which is set about 3 feet in the ground. A third style of bench mark with abbreviated lettering (b and D, Pl. I), is used for unimportant points. This consists of a special copper nail, 1} inches in length, driven through a copper washer seven-eighths Of an inch in diameter.
The tablets as well as the caps on the iron posts are appropriately lettered, and for cooperating States the fact Of such cooperation is indicated by the addition of the State name (at G, P]. I).
The numbers stamped on the bench marks described in the follow ing pages represent the elevations to the nearest foot, as determined by the levelman. These numbers are stamped with Tag-inch steel dies on the tablets or post caps, to the left Of the word feet. The Office adjustment of the notes and the reduction to mean sea level datum may so change some Of the figures that the original markings are 1 or 2 feet in error. It is assumed that engineers and others who have occasion to use the bench-mark elevations will apply to the Director of the United States Geological Survey at Washington, D. C., for the adjusted values, and will use the markings as identification numbers only.
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