Architects indicate grade elevations on a site plan by first establishing a "benchmark," which is an existing reference point that should remain undisturbed during the course of construction. The benchmark may be a sidewalk or a steel stake driven in the ground, and architects often assign an arbitrary value of 100.00 feet for the benchmark's elevation. All the other grade elevations are then calculated relative to the benchmark. For this information to be useful to construction workers, the decimal part usually has to be converted from 1/100s of a foot to inches.
Subtract the elevation in question from the benchmark's elevation. Ignoring the decimal value, the difference is the number of feet below the benchmark. Negative values indicate elevations above the benchmark. For example, given a benchmark at 100.00 and a grade elevation at 101.43, the difference is -1.43, which indicates that the elevation is 1 foot and some odd inches above the benchmark.
Multiply the decimal part of the difference times 12 to find the number of inches. For example, 0.43 x 12 = 5.16. Ignoring the decimal part for now, that indicates that the elevation is 1 foot 5 inches above the benchmark.
Multiply the decimal part of the inches value times 8 to find the number of 1/8 inches. You could multiply by 16 to find 1/16s, but that much accuracy is unnecessary for building elevations. For example, 0.16 x 8 = 1.28. Round that off to 1, giving 1 foot 5 1/8 inches for the final elevation.
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