How Things Work: Basic Total Station Calibrations
Professional Surveyor Magazine - May 2002
This month our focus is on the basic adjustments of a total station. We recognize that many organizations do not encourage field crews to actually adjust the instruments. Whether it is through policy or unavailability of the proper circumstances to effect adjustments, total station users should still be aware of the basic calibrations, and most important, know how to test and evaluate the level of calibration. This column focuses on the calibrations for circular and tubular (plate) vials, optical plummet, horizontal and vertical collimation, and the trunnion axis. In most cases these adjustments can be checked with every setup of the instrument.
Tubular (plate) Vial
The purpose of the plate vial is to ensure that the vertical axis of the instrument is truly vertical. Then, if all the other adjustments are correct, the horizontal axis and horizontal circle will be truly horizontal, and the telescope, when rotated, will describe a vertical arc. With an instrument that has three leveling screws (A, B and C), the axis of the tubular vial must be placed parallel to the line connecting two of the screws (A and B), and the bubble must be centered using one or both of the leveling screws. The alidade must then be rotated 90º so that the axis of the tubular vial is at right angles to the original position, and the bubble must be centered with the one remaining leveling screw (C). Good practice requires that the centering of the bubble be checked and adjusted with the leveling screws at the first position again, and then the second position. The instrument cannot be assumed to be level at this point. The adjustment of the vial must first be checked. To do this, the alidade is now rotated so that it is 180º from the first or second positions. Any apparent movement of the bubble indicates twice the amount of error in the adjustment of the vial. To properly use the instrument, adjust the position of the bubble to move it back half way to the center. Repeat this in the remaining position, which is 180º from the other position where the bubble was centered. This process finds the reversing point of the bubble tube. When the bubble's position does not change in all four principal positions, the instrument is level.
The only purpose of the circular vial is to enable the user to roughly level the instrument so that leveling with the tubular vial can be done. If this vial is damaged or out of adjustment, there are no serious consequences when correct surveying procedures are followed. After the instrument has been leveled, a perfectly centered circular vial tells you that it is in adjustment. If not centered, the displacement from center reflects the actual error.
When truly level, a well-calibrated optical plummet's line of sight is along the vertical axis of the instrument. This is easily checked even if the instrument is not level. Simply sight through the plummet and place the cross hairs on a point or mark where the intersection of the cross hairs fall. Then rotate the alidade 180º and check that the cross hairs fall on the same point. If they have moved from the original point, the apparent movement represents twice the error in the optical plummet. The instrument can still be centered by ensuring that the ground point falls midway between two positions of the optical plummet's cross hair.
Horizontal and Vertical Collimation
Horizontal and vertical collimation are calibrations that ensure that the line of the sight through the telescope, as represented by the vertical and horizontal hairs of the reticle, are truly horizontal and vertical. A simple check may be done in the following manner: Sight a clearly defined point in Face 1 approximately 200 feet away. Then record the horizontal and vertical angles. Now invert the telescope so that it is in Face 2 and record the angles again. Because of random error in the observations, repeat this process a half-dozen times. Now compare the average of the angles in Face 1 with the average of the angles in Face 2. When in proper collimation, the difference of the horizontal angles will be exactly 180º, and the sum of the vertical angles must be exactly 360º. Small differences of one or two seconds are inconclusive in determining whether the instrument is in or out of calibration. Large differences (the magnitude of "large" will vary depending on the instrument and the type of work it will be used for) will require that an equal number of Face 1 and Face 2 observations be done for each point observed to ensure that the collimation errors cancel out. If it is planned to use the instrument for a large number of single face observations such as topographic survey, it may be necessary to have the instrument calibrated before using it.
The test to ensure that the trunnion or horizontal axis of the instrument is in adjustment is sometimes referred to as the "height of standards" test. The test is very simple, and facilitated by the use of a level rod. Set up the instrument and level it so that it is possible to sight a clearly defined point approximately 200 feet away at a steep sighting angle (the top of a church spire is a good example of such a point). Now depress the telescope so that you are sighting at the ground, as close as possible to the structure or object you have sighted. Place the level rod on the ground so that the vertical cross hair cuts through the rod, and note the rod reading. Now invert the telescope and sight the elevated point again and depress the telescope so that the rod is visible. If the vertical cross hair cuts the rod at the same point as in Face 2, the trunnion axis is well- calibrated. Any deviation reflects twice the error in the verticality of the line described by the vertical cross hair. If it is being used to vertically align objects, it must only be used by observing in Face 1 and Face 2 and using the mid-line for alignment.
Thanks to Matt Delano with Nikon for assistance with this article and for the images.
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