introduction
Differential Leveling Basics
Differential leveling is a very simple process based on the measurement of vertical distances from a level line. Elevations are transferred from one point to another through the process of using a leveling instrument to read a rod held vertically on a point of known elevation, and then, on the point of unknown elevation. Simple addition and subtraction are used to calculate the unknown elevations. Perhaps this procedure can best be understood through a few definitions and illustrations.
Elevation
Elevation is the vertical distance above a zero datum. Datum in the case of leveling is generally based on the ocean’s Mean Sea Level (MSL). An elevation in the mile-high city of Denver would be approximately 5280 feet. An elevation in the city of New York, which is close to the ocean level, might be 15 feet. A point in Yellowstone might be 8900 feet above MSL. There is a network of MSL monuments around the United States which have been established by the U.S. Government.
If you are at a construction site where there is no MSL monument with an elevation nearby, a datum point is established and an elevation is assigned. Generally, a number large enough to prevent any negative elevation numbers from occurring is picked. Usually 100.00, 500.00, or 1000.00 is used. If, at a later time, an MSL elevation is available, it is easy to convert any assumed elevation to MSL by determining the difference at the established datum point and adding that to each established elevation.
Benchmark (BM)
A benchmark is best described as a permanent, solid point of known elevation.
Benchmarks can be concrete monuments with a brass disk in the middle, iron stakes driven into the ground, or railroad spikes driven into a tree, etc. See Setting Benchmarks on Page 7-48 later in this Chapter.
Backsight (BS)
A backsight is a reading on a rod held on a point whose elevation is known.
Height of Instrument (HI)
The height of instrument is the elevation of the line of sight of the instrument. This is determined by adding the backsight to the known elevation.
Foresight (FS)
A foresight is a reading on a rod held on a point whose elevation is unknown.
Turning Point (TP)
A turning point is a point used in the differential leveling process on which to tem-porarily transfer the elevation from the HI. The elevation of the TP is determined by subtracting the FS from the HI.
Closed Loop
Closing a loop means to end the level loop at the benchmark where you began or at another benchmark. "Always, always, always close your loop" is a rule of thumb which must be strictly observed in leveling. If a loop isn’t closed, there is no way of knowing if any mistakes were made in the leveling process.
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