Point of Beginning

GIAA Mailbag

March 1, 2005
Tool tips on tripod maintenance and reflectorless EDM technology.

I was talking to my instrument dealer about the accuracy of my total station, and he asked me if I do maintenance on my tripod regularly. I confess I don't know what I should be doing to maintain my tripod. Can you help?

Tripod maintenance is important to ensure that you get the accuracy your total station is capable of delivering. The potential for movement of a tripod after an instrument has been set up over a point and leveled is aggravated by heavy total stations that are servo-driven. Rapidly starting and stopping rotation of the instrument adds torque to the list of factors that can cause inadequate tripods to be the cause of measurement errors. Further, most adjustment routines inadequately interpret such errors and correct for them. You should check the tripod screws and bolts anywhere from weekly to monthly. Hinges should be checked for stiffness. To do this, fully extend all the legs of the tripod. Then, one by one, raise each leg until it is horizontal. Then let go. The ideal stiffness is when the leg gently drops to the ground. Leg tips or points should be checked to make sure they are not loose. The dowels on the upper part of the leg where they insert into the hinge assembly should be securely attached. You may need to ask your instrument supplier to show you how to tighten these, as there are many methods of attachment. The clamps used to tighten the sliding part of the legs should be lubricated, and the mechanism should be checked for wear. It shouldn't take a lot of turning to tighten and loosen the legs to extend and retract them. If it takes a lot of turns, the problem could be in the parts that are not generally visible. You may have to disassemble the tripod leg completely, and if needed, get replacement parts. "Stickiness" when extending and retracting the legs could also occur from damage that has roughened the part of the legs that slide. If your tripod is wooden, touch up the places where the paint has been removed. Use good quality coating and allow enough time for curing before using the tripod again. Finally, visually check and run your fingers over the tripod's head surface. It should be smooth. If it isn't, it probably is because you are not using the protective cap supplied by most manufacturers. Lack of smoothness on the head could cause difficulty in leveling. Unless you are an experienced metal worker, ask your qualified instrument repair shop to smooth it for you.

We have a reflectorless total station. For stakeout work it works well to use it with a short (3 ft or less) prism pole with a target but no prism. Keeping the pole plumb is much easier without the heavy prism. We press a disk of plastic into the part of the target that has the cup and screw for mounting a prism. We've now covered that plastic with a sheet of reflective material. But what prism constant should we be using? I don't think it should be zero.

Figure 1. Cross section of prism holder with reflective disk behind pivot point.
It seems that you are doing a good job of taking advantage of reflectorless EDM technology. The ideal target position for the type of application you mention is one where the plane of the reflective surface is along the plumb line, with a pivot around a horizontal axis passing halfway through the surface. You may look at some of the old tilting targets that were made for use with add-on EDMs that were mounted on top of theodolites. For the setup you describe, set the prism holder with the reflective surface such that the axis of the mounting hole for the prism is horizontal if the prism pole is vertical. Then use a scale to measure from the reflective surface to the plane, which contains the vertical line running through the middle of the prism pole (see Figure 1). If this surface is farther away from the EDM than the middle of the prism pole, the prism constant will be negative. If it is closer than the middle of the prism pole, the prism constant will be positive. Assuming this is a tilting prism assembly, you may have to account for tilting of the housing if the pivot is not along the plane normal to the incident beam from the EDM.