GIAA Mailbag: On Right Standards and Backsight Horizontal Circle Values
A: The standards of an angle measuring instrument (transit, theodolite, total station) are the parts of the alidade of the instrument that are upright. They support the horizontal axis upon which the telescope is mounted. There is some type of bearing at each end. With the telescope in what is called “direct” position in U.S. surveying jargon, the right and left standards are those to the left and right of the user of the telescope. Use of the instrument with the telescope in the so-called “direct” and “reverse” positions does not change the designation of these standards. That is, from the point of view of the telescope user, the “right standard” is on the left side when the telescope is in reverse position. Because in the days of the optical telescope it was traditional to locate the vertical circle in the left standard; international nomenclature for the “direct” telescope position has been “circle left” and “reverse” telescope position has been “circle right.” Other names manufacturers use for these positions, particularly since the introduction of electronic instruments have been “position I” and “position II” and “V1” and “V2” for “direct” and “reverse,” respectively. With respect to the adjustment itself, when all other adjustments of the instrument have been checked and adjusted, and the instrument has been leveled, it may still be possible that as the telescope is elevated or depressed that the crosshair tracks an inclined line. This may indicate that there is a “height of standards error,” or in international jargon, “trunnion axis error” that needs to be accounted for or adjusted. In a transit (i.e. steel circle instrument), a knowledgeable user could turn the appropriate capstan screws in the right standard to bring the instrument back into adjustment. Most optical and electronic instruments, however, need to be repaired by technicians trained in the technology used in the product. You will find most good textbooks intended for university level courses to give a thorough explanation of this problem and its correction. (See also GIAA Mailbag’s Tool Tips column in POB’s November 2002 issue.)
Q: Does it make any sense to vary backsight horizontal circle values between sets to eliminate horizontal circle eccentricities in electronic theodolites, as was the practice with repeating (i.e. upper and lower motion) transits of yesteryear? In Massachusetts, for instance, one is required on certain public projects to backsight with 0º for one set and then 90º for the second. I argue that one has changed nothing except the angle setting. The instrument’s circle is still oriented exactly the same.
A: You could be right! The reason for advancing the circle between sets of angles whether with repeating or direction instruments is to distribute measurements around the entire circle, thus causing circle eccentricities to be averaged. To determine whether there is any benefit to setting the horizontal circle values as you describe, you must examine your procedures as well as the specifics of the circle reading system used by the manufacturer of your instruments. The answers could vary between instruments and between processes. On so-called incremental reading electronic theodolite circles, there could be a zero value that is absolute, that must be detected by the instrument after switching it on. If this is done, and if the starting angles (i.e. backsight circle reading), in a two-set observation are 90º apart, then you have distributed the measurement around the circle. Some instruments however, set themselves to 0º at whatever point they are when switched on. To distribute the angle over 90º, be sure that the instrument is not switched off between sets; this will cause the ability to determine the orientation of the second backsight angle with respect to the first to be lost. Some instruments, however, do not have the capability to rotate the circle; in these cases surveyors have been known to set the backsight angle by keying in 0º for the backsight on the first set and 90º for the second set. This does not distribute the readings over the circle. The only way to rotate the circle is to rotate the entire instrument on the head of the tripod. An approximate way of doing this is to rotate the instrument to the right the required number of degrees (90º if you are doing two sets), after you finish taking your last circle reading on the first set. Then clamp the telescope. Loosen the tripod fixing screw and rotate the entire instrument, including the tribrach until the telescope is pointed at the last target. Now point to your backsight after re-leveling and re-centering and complete the set. Certain instruments use the entire circle for every measurement of a circle reading. They do this in a variety of ways, including rotating the circle for every measurement. If you are unable to determine on your own whether the circle is fixed, fixed but can be rotated by you, fixed and completely considered for each measurement, or rotated with each measurement, contact the manufacturer for details.
Illustration courtesy of Trimble.
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