In the previous column (September 2007), we began a discussion of errors inherent in optical plummet tribrachs. We conclude that discussion with this column.
A. We’ve already discussed the topics of tribrach circular bubble vial sensitivity, determining average leveling error when using the circular vial only, and the calculation of errors in centering and horizontal angle measurement. Now we cover some ways to reduce those errors.
Factors affecting how well an optical plummet tribrach is leveled.First, evaluate how well you can level each optical tribrach with the circular leveling vial only using the process discussed in the previous column. You must then decide whether this error is acceptable or whether you need to take other steps to reduce this error to something more tolerable. This evaluation and analysis needs to be done with each tribrach because the level bubble sensitivity varies from manufacturer to manufacturer and sometimes even from model to model (including year of manufacture). It is a good idea to re-evaluate this error periodically as the tension in the leveling screws can also affect how well you level the tribrach. Each person who levels the tribrach may have a different visual perception of the bubble, so the amount of error contributed by the optical tribrach leveling process may also vary. Finally, the shape of the actual equipment in the tribrach (prism, total station, target, antenna, etc.) could restrict the view of the level bubble and reduce the operator’s ability to determine visually if the bubble is correctly centered.
Reducing impact of centering.With the level vial adjusted, the principal way to improve centering with an optical plummet tribrach is to constantly monitor and adjust the optical plummet. You can find many methods for doing this check (see any good surveying text for specifics). Arguably, the best method for evaluating optical plummet adjustment is done indoors.
As the illustration below shows, set up a tripod on a solid floor, and make sure it is stable. Now, attach a tribrach (not the one being checked) to the tripod. Now, place a tribrach adjusting ring (sometimes a solid block) in the tribrach. It is designed so that it fits snugly into the tribrach. Now, place the tribrach to be checked upside-down on the tribrach adjusting ring. Look through the optical plummet (you will be looking at the ceiling), and first adjust the cross hairs. Then bring the ceiling into sharp focus. Sight a point on the ceiling (you can set or mark a target if you like). The best way to aim the cross hairs at the target is to use the leveling screws of the fixed tribrach. (Note that the tribrach does not have to be level to perform this check.) Once on the ceiling target, rotate the tribrach to be adjusted 180 degrees. If the cross hairs still remain on the target, the optical plummet is in correct adjustment. If not, the discrepancy represents twice the optical plummet’s sighting error. Adjust the cross hair (following the tribrach manufacturer’s recommendations and tools) to halfway between the target and the position of the cross hair after it has been rotated 180 degrees. Always repeat the test until no visual error is detected.
For surveys exceeding the capability of the tribrach’s level bubble and optical plummet.Many high-precision surveys require better leveling and centering. Because the limitations of the optical plummet tribrach are due to the fact that the level vial and tribrach cannot be rotated about the vertical axis (as in a total station with a level vial and optical plummet in the alidade) as well as the relatively low sensitivity of the circular level vial, the only option to improve leveling and centering is to not use an optical plummet tribrach. One option commonly used on high-precision surveys is to use nonoptical plummet tribrachs together with a tribrach adapter that has a rotatable optical plummet and more sensitive tubular level vials. These adaptors can be used with targets, prisms and GPS antennae and are available from many suppliers.
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