I have called instrument manufacturers and asked my local supply guys about “horizontal compensators,” but I am still unclear about proper field procedures for using total stations with these devices. How do I get optimal results using an instrument equipped with such a device?

Compensators on total stations are used to correct for the tilt of the vertical axis of the instrument. This tilt is due to a variety of causes including small imperfections in leveling at the time of setup, small settlements of the tripod after setup, and uneven heating of the instrument from solar radiation. Early instruments with electronic or opto-mechanical compensators measured the tilt of the

vertical axis only in the direction of the telescope axis, and so only corrected the angles in the vertical plane (zenith angles). Some called these compensators “vertical compensators.” Newer instruments have so-called two-axis compensators that also measure the tilt of the vertical axis perpendicular to the telescope axis or parallel to the precise leveling vial (often referred to as the “plate” vial). The purpose of measuring vertical axis tilt in a direction perpendicular to the telescope direction is to correct horizontal angle errors. Tilt in the direction of the telescope affects the angles measured in the vertical plane, and tilt normal to the telescope axis affects the angles measured in the horizontal plane. Horizontal compensators have the ability to sense the tilt normal to the telescope axis, which is then fed into the instrument’s processing system so the displayed angle is corrected.

The optimal field procedure to assure that the compensator is properly adjusted is to perform the checks and/or adjustments recommended by your manufacturer (you can read your manual and do some of these yourself on a daily basis). Because of the compensation features, you may see the horizontal angle change even when you have the horizontal motion clamped as you elevate and depress the telescope. This change should be noted when performing functions where the track of the vertical cross hair is critical, such as aligning structures intended to be vertical.

There are some situations where it is desirable to switch off the compensation and rely instead on a rigid setup, after first making sure the instrument is shaded and effects of wind have been blocked.  However, these are unusual situations and it is wise to consult with someone experienced in these procedures before doing so. Any comprehensive text on surveying will have a discussion of the horizontal angle correction. Sometimes the discussion may be found in the section on astronomical or geodetic observations since the errors get larger as the telescope is tilted away from the horizontal plane.

The EDM on my total station is rated in the manufacturer’s literature as being ±(3 mm + 3 ppm). What does this mean in practice?

You have to check with each individual manufacturer, but generally the accuracy of the EDM is the standard deviation of a single measurement. For those not familiar with this concept, standard deviation, often represented by the lowercase Greek letter “sigma” (s) is actually a representation of uncertainty at approximately the 68 percent confidence level. This means that if a single measurement is made, there is a 68 percent probability that the measurement result will be within the published rating.

Those familiar with this concept will also know that the 95 percent and 99.9 percent confidence levels are easily determined by multiplying s by 2 and 3, respectively. The statistical basis for stating accuracy is actually determined by measuring precision, but only after all known systematic errors (instrumental, natural and human) have been removed. In your case of ±(3 mm + 3 ppm), the expected error in a 100 ft measurement at the 68 percent confidence level would be 0.01 ft. The corresponding 95 percent and 99.9 percent errors would be 0.02 and 0.03 ft. Interestingly, these correspond to relative accuracies for the measurement of 1:10,000, 1:5,000 and 1:3,333. As the accuracy is dependent on the line, the same accuracy rating would result in an error of 0.011 ft. Precision for this measurement at the 68 percent, 95 percent and 99.9 percent levels would be 1:45,000, 1:23,000 and 1:15,000, respectively. Remember, these are theoretical numbers based on manufacturers’ ratings and how your equipment is used.

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