With a 5 arc-minute error in the mechanical axis, the plumb line through the optical plummet is shifted 0.006 feet.

Q: I really appreciate the modern total station because it takes care of so many things I used to worry about … like the two-axis compensation system that corrects leveling errors. But sometimes I wonder: Is there a downside to all these wonderful inventions?

A: Modern total stations do have a number of routines to automatically detect and correct anomalous situations that are well within the normal range of error sources we encounter in surveying. But it is important to understand how they work and what they correct. The two-axis compensation system is a good place to start.

First of all, it is called “two-axis” because it detects tilt in the two principal directions of the instrument’s local coordinate system. The x-axis is in the direction of the line of sight, i.e., wherever it is pointing in azimuth. The y-axis is normal to that; think of it as the “side-to-side” axis.

X-Axis Tilt Detection and Correction

The purpose of detecting tilt in the x-axis direction is primarily to determine the correction to be applied to the observed vertical circle reading. The vertical circle is oriented so that zero degrees on most instruments is along the upward direction of their mechanical vertical axis.

The process of leveling the instrument is to align that mechanical vertical axis with the true vertical--the direction of gravity. If the mechanical vertical axis is slightly out of plumb when a vertical circle reading is taken, then the error in the vertical circle will have a component that is exactly equal to the out-of-plumbness in the direction of the line of sight. (There are other errors possible when a vertical circle reading is obtained, but we won’t get into them here.)

The mechanical vertical axis may be out of plumb for several reasons. The plate bubble may be out of adjustment; the ability of a person to center the bubble has limitations; the instrument may have settled slighty after intially being leveled; solar radiation, wind or wind-caused vibration or cooling may be causing movement; or expansion or contraction may have occurred in the tripod or even the instrument itself. These and other incidents can induce small leveling errors.

Y-Axis Tilt Detection and Correction

The purpose of detecting tilt in the y-axis direction (normal to the line of sight) is to correct horizontal circle readings. The reasons the small errors in leveling occur are similar to those discussed under x-axis tilt detection and correction. In addition to using the tilt in the y-axis direction, the inclination of the telescope is also used to compute the correction to be applied to the horizontal circle reading.

Big warning: As you approach the limit of range of “out of tilt” in which the compensator will work, be aware that errors in plumbing over the ground point may be induced. If an instrument is not truly level, then the line of sight passing through the instrument through the optical plummet to the ground is correspondingly out of plumb. Surveyors must evaluate this error in centering for each instrument to know when to limit the automatic tilt compensation correction to horizontal angles.

This limit will vary depending on the accuracy a particular survey is to achieve. For example, with an instrument that has a +/-5 arc-minute compensation range and a setup that is 5 feet above the ground point, the shift in the optical plummet’s line of sight away from the vertical is approximately 0.006 feet at the maximum of 5 arc-minutes. That is not a large amount, to be sure, but you should be aware of it and budget for errors accordingly.

A Note About Compensators in General

A good tilt-compensation system can correct both horizontal and vertical angles. But the efficacy of the tilt compensation depends on the tilt-detection system being in good adjustment. Just as with plate vials, tilt-compensation systems can go out of adjustment. In fact, many manufacturers recommend frequent checking and adjusting of the compensation system (often more than once per day, especially if there are large ambient temperature changes). It is impertative that someone in your office read and understand the process by which to check and adjust the tilt-compensation system. Then teach every field-crew member who has responsibility for good instrument operation.

Sometimes, users relegate checking and calibration of their instruments to semiannual or even annual activities. The truth with modern electronic instruments, especially total stations, is that such a long interval is not advised. A careful read of your manual will help you understand when and how checking, calibrating and adjusting certain aspects of your modern technology are to be performed. By following these recommendations, you will get much better service out of your instruments.

Send your questions for GIAA Mailbag to giaamailbag@gmail.com. Answers will be published in future issues of POB.

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