A level rod is a measuring tool used by surveyors to calculate the difference in elevation between two locations. They can be adjusted for height and come in different materials. There are two types of level staff, target rods and self-reading sticks, and they have graduated number markings. The measurement is taken using a theodolite and a vertical staff, and the difference in elevation is calculated through triangulation.
A level rod is a measuring rod that surveyors look through a telescope to calculate the slope, or difference in elevation, between two locations. Whatever their material construction, most all models can be adjusted for height, telescopically or with additional sectional accessories. Also called leveling bar, its use is based on one of the simplest principles of geometry.
They may be a single barrel long, but land surveying on foot requires a certain degree of portability. A lightweight aluminum level staff may have nested sections that extend and lock for height adjustment. Wooden tipis can be made in separate sections that connect end to end and lock if necessary. All have graduated number markings, imperial or metric measurements, in large, visible print. They are, basically, very long rulers with a functional resolution of about 0.1 inch (2.5 mm).
Furthermore, there are two different types of level staff, which correspond to who is determining the measurement. A “target rod” is read by the rodman, the individual who holds the staff level. Another type, a “self-reading stick,” is read by the instrument man. The latter, sometimes called a ‘talking bar’, is sighted and recorded by the individual looking from a distance through a telescope mounted on a tripod.
Target rods are mounted with a sliding target marked with a crosshair and cut with a keyhole in the center. With hand signals or by radio, the tool man instructs the rodman to slide the target up or down until his sight matches his scope’s sight. These are also sometimes called stadium marks.
There are a few other staffing levels that are designed differently for specialized applications. A surveyor’s rod has a zero point in the center and is useful for continuously recording the advantages and disadvantages of the wildly undulating elevation contours of an area of land. So-called standia rods are oversized versions of a dipstick used for long distances.
The geometry of the elevation change measurement is quite simple. The pointing tool most surveyors use is called a theodolite, and the adjustable legs of its tripod mount can be adjusted to ensure that the telescopic sight is truly level. Its height above the ground is recorded.
At a practical distance of up to 410 feet (125 m) for most models, a vertical staff is displayed to record the measurement indicated by the telescope’s viewfinder. The difference between the two measurements is the change in elevation. If the exact distance is also recorded, a simple triangulation calculation provides a measurement of the change in elevation in degrees or slope.
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