GIAA Mailbag

November 1, 2004
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Tool tips on tapping automatic levels and the definition of a surveying system.

Photo compliments of Sokkia.
Q: When I was taught to use automatic levels, I was told to tap the instrument lightly before taking a reading. Why? Isn't there a possibility that I could disturb the reading and affect the accuracy of the survey?

A: Automatic levels work on the principle of a movable optical component, usually a prism, or a mirror that is suspended and responds to the pull of gravity, orienting itself to assure a horizontal line of sight through the instrument. When automatic levels were first introduced and before the technology was mature, the development of almost frictionless suspension systems for these optical components was ongoing. In testing, it was discovered that not quite horizontal lines of sight were occasionally established due to friction, preventing the optical components from coming to rest where they should. Many surveyors developed the habit of tapping the instrument with a finger just prior to taking a reading. This was to cause the optical component to vibrate slightly, and if it hadn't been in the correct position, to help it get there. Today's levels have much improved suspension systems and rarely, if ever, need such a helping tap. If you do choose to tap the instrument, tap it lightly enough to be sure the reading isn't disturbed. By checking the rod reading before and after tapping you will develop a feel for applying the right amount of force without causing error. Even if the prism "sticks" it shouldn't be happening with every reading. If you observe such a condition, discontinue use immediately and take it to a qualified repair shop for evaluation and servicing.

Q: Many people make reference to a surveying system. Why do they use this term instead of "instrument"?

A: A surveying system refers to everything that goes into making measurements. It not only includes the instrument, but also the accessories (tribrachs, prisms, plummets, prism poles, tripods, etc.) as well as the people who make the measurements. Ideally it should also include the processes used by these people. This change in reference has occurred because when planning work or reviewing it, consideration needs to be given to the whole system, not just the primary component (the instrument). We run into many cases where surveyors will complain or investigate larger than expected errors by examining or complaining about the performance of an instrument. However, it can be the accessories or the processes used to survey that may be the culprit. There are several common errors we have found, such as setting up a tripod that is not sufficiently stable to support the instrument. In the case of robotic instruments, it may also be that the tripod itself is not strong enough to support the direct load of the weight of the instrument, and perhaps even the significant torques created by the servo motors. Prisms with the wrong offset are another common error that may be blamed on the instrument. Incorrectly adjusted optical plummets or tribrachs can also be the source of significant measurement errors. Bent prism poles or circular bubbles on them that are out of adjustment are yet another cause of error that is often overlooked.

Similar processes can be inadequate to get the desired accuracy. Overlooking the temperature and barometric pressure with EDM (i.e. ignoring the PPM correction) is a common process error. Lack of care in measuring heights of prisms, instruments and antennas is another way surveys can be ruined. It can also be that the instruments and accessories are not being used in a way consistent with the manufacturer's recommendation for getting a particular level of accuracy. Reviewing the instrument and all the accessories-and determining what adjustments if any should be checked or adjusted-is a good way to start eliminating sources of error in the system. If you are not sure how to do this, consult with your surveying equipment distributor.

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