GIAA Mailbag: On Prism Configuration and Checking the Adjustment of Leveling Devices
Q: We use a triple prism cluster that is triangular, but have noticed a stack of three arranged vertically, or even horizontally, used by other surveyors. Is one configuration better than another?
A: First, triple prism configuration must be what your manufacturer recommends. Some manufacturers (not all) specify a triple prism shape that best suits the infrared beam emitted by the EDM. If none have been suggested, any shape will probably work. However, keep in mind that the triangular configuration is more compact; it fits into a circle smaller than a circle that a horizontal or vertical stack would fit into. This is important, as you must make sure that the EDM strikes all three prisms. Check your manufacturer's specification for the width (or diameter) of the EDM beam. Triple assemblies must be used such that when the EDM is pointed at their (combined) center, the beam emitted goes beyond the edges of the assembly. A good practice to follow is to only use a single prism, even if you have to use the triple assembly, until you are so far away that you have exceeded the single prism range of the instrument. If you only expose the other two prisms at that point, you can generally be assured that the beam overlaps all three prisms, regardless of the shape of the assembly. What you must make sure to avoid is partial exposure of one or more prisms to the EDM beam. Such a situation can cause errors in the distance measurement. You can illustrate this to yourself by conducting a prism sweep test. Set up your instrument and a single prism about 250 ft away. Point at the center of the target, take a measurement and record it. Then use the horizontal or vertical tangent screw to move the cross hairs perhaps one arc-minute off-center and take another measurement. Repeat this process, increasing the angle away from the center of the prism by equal increments. You will start to see systematic variations in the distance measured. For a complete test, you should go up, down, right and left of the prism's center.
Q: I have an electronic level on my total station as well as a spirit bubble. Which should I use?
A: As long as both are in adjustment, either one will work. However, you may want to check your manufacturer's specifications for both. It may be that one is more sensitive than the other. Then, depending on what you are doing with your instrument, you can use your judgment to decide which one to rely on. There is a tendency to overlook the possibility that an electronic level can go out of adjustment. Surveyors are generally used to the idea that a spirit bubble can go out of adjustment and most know how to check it: center the bubble carefully in one direction, rotate the alidade 180Âº and if the bubble is not at the center, the apparent movement represents twice the error in the adjustment of the level vial. This principle is just as true with the electronic level and in fact is checked the same way, except that the apparent movement of the bubble is read out digitally as arc-seconds of tilt. You will then have to refer to your instruction manual on the procedure to adjust it. Just like spirit bubbles, electronic levels are easily affected by changes in environmental conditions, and should be checked and used with a similar degree of care. It is good practice to check the adjustment of either type of leveling device whenever you set up your instrument.