A bench mark (BM) is an object whose elevation above mean sea level is known or assumed to be known. A bench mark must be an object that is dimensionally stable because it is the reference point for all of the elevations for a survey. If the bench mark elevation is accidentally changed, all surveys that used it must be redone. Bench marks allow a survey to be repeated at a later date and permit a surveyor to tie elevations from the current survey to elevations established in previous surveys. A network of bench marks can thus be established over a large area, all tied to the same reference elevation. Bench marks may vary in character and permanency according to the survey for which they were established. The U.S. Geological Survey (USGS) has established a nationwide network of bench marks, all referenced to mean sea level. These marks consist of bronze disks set in concrete monuments, similar to right-of-way markers, which have been firmly set in the ground. The date of the survey and the elevation and bench mark number usually are stamped in each bronze disk. The adoption of Global Positioning Satellites (GPS) for surveying has provided another means of establishing benchmarks.
In many situations it is not necessary to know the exact elevation above sea level. For such surveys, a local bench mark is used. Frequently, this bench mark is given the elevation of 100.00 ft. If the terrain is hilly, the surveyor should choose a larger number for the starting elevation, as it is not standard practice to use negative numbers for elevations in common surveys.
When using a local bench mark, the survey crew must select and establish its location. Two rules should be followed. The object selected should: (1) be reasonably permanent for as long as it will be needed, and not easily moved or otherwise destroyed; and (2) be capable of being described in such a way that it can be easily relocated. A typical local bench mark might be an "X" chipped in a concrete curb or a bridge abutment, an iron pin driven firmly into the ground, or the rim of an electrical or sewer access hole. It is the job of the note keeper to describe accurately the name, number, type, elevation, and location of each bench mark, and to record this information on the right-hand page in the surveying notebook.
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