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A Philadelphia rod is a type of leveling rod used by surveyors. Made of metal or wood, it consists of two sections each measured off in specific gradients. The two sections are arranged side-by-side and are joined together at one point by a small metal sleeve through which the pieces can slide. This allows the Philadelphia rod to be used at greater or lesser extensions, which enables it to be functional over a wide variety of types of terrain and over shorter and longer distances. The ability to be extended and retracted, which also makes it easier to transport, is the reason many consider the Philadelphia rod to be the most popular survey leveling rod.
Surveying is defined as viewing, measuring and correctly recording specific positions so that fixed points can be defined for mapping, accurate boundaries can be created and a reading of the grade of the slope of the land can be determined for use in construction. To accurately survey, the survey team needs a fixed point of observation, a survey leveling rod held vertically at a second point and a reading device, much like a telescope combined with a level, with which the surveyor can site from the first fixed point to the leveling rod.
Construction of a wooden Philadelphia rod often uses maple painted white with black numerals and gradations. Metal Philadelphia rods often are made of aluminum with gradations stamped in and also painted black. It is traditional to have a brass sleeve through which the two sections can slide. In both types of rod construction, extremes of heat or cold can cause the metal or wood to expand or contract and can affect the accuracy of readings.
Philadelphia rods use both metric or U.S. standard measurement systems. If metric, the rod is graded off in meters and centimeters, with a bold mark at each 10th of a meter point. If measured in feet, the Philadelphia rod is graded off in 100th of a foot segments with a bold mark at each 10/100ths of foot. The standard Philadelphia rod has two 7-foot (2.134 meter) sections and can be used to a height of 13 feet (3.96 meters).
Improvements to the Philadelphia rod include removable graded face plates. Extensive use over time can wear and blur the numerals and gradations. Removable face plates can be replaced as needed to keep gradations visible.