GIAA Mailbag: On Tripod Adjustments and EDM Retroflectors
Q: I know quality, well-adjusted tripods are more conducive to accurate surveys than poorer quality, unadjusted ones. What should I look for quality-wise and what adjustments are critical?
A: Whether to support targets, antennas or optical instruments, tripods form an important component of the system you rely upon to make required measurements. Quality tripods may be made from a variety of materials including, wood, metal, mostly aluminum and composites such as fiberglass. You should understand how much trading off weight and cost may affect the stability of your setup. This can only be evaluated by knowing the tendency of the tripod’s legs to lengthen or shorten with changes in temperature. This may occur from differential changes when the tripod is exposed to sunlight, which tends to cause rotation of the tripod head. Other changes that affect the stiffness and effectiveness of the joints and clamps may also mis-adjust the tripod. This information may be available from your dealer or the manufacturer. Maintenance checks should include: making sure the legs of telescoping tripods slide smoothly, that the head joints are stiff, that the clamps tighten and loosen effortlessly and smoothly, and that the points and shoes are not loosely attached.
A: EDM prisms are generally designed to provide stable distance measurement over their life. However, the construction method of some prisms may allow the glass within the housing to move in or out. Periodic visual inspection of the prisms is advised. Movement of the prism may be detectable this way. A better way to check for adjustment is to mount the prism in a single prism holder and use it in a “three-peg” test procedure that determines the prism constant (see figure). On relatively flat ground set up your EDM near the middle of an approximately 300 ft long line. Measure from C to A and C to B. It is advisable to use tripods rather than range poles to support the prism. Be absolutely sure that the optical plummets used with the instrument and prism are in proper adjustment before performing this procedure. Now move the instrument to one of the end lines and measure to the prism at the other end. The sum of AC and BC should equal AB. If they don’t, the difference is the error between the true prism constant and the value set in the EDM. When and if a change occurs, make sure you have eliminated faulty optical plummets and faults or changes in the instrument. When you inspect your prism, also check for chips or cracks in the glass. They should be discarded if such damage appears. Also use a good quality lens cleaner and lens cleaning tissue to clean the front surface of the prism. Disassembly of the prism is not recommended except by qualified repair shops.
Have a question for the GIAA Mailbag? Please E-mail your questions to Diana Brown at firstname.lastname@example.org. Although we can’t print all of the questions, we will attempt to cover as many as possible. You may also ask your questions or continue the discussion at www.rpls.com.
Berntsen International Inc.
Leica Geosystems Inc.
Topcon America Corporation
Professional Surveyor magazine
Tripod Data Systems Inc.