Tool tips for reducing errors with multiple angle measurement.
A: These terms have come into use in the United States partly because they are terms used in the home countries of many theodolite and total station manufacturers.
“Face” refers to the vertical circle, which has traditionally been located on the left standard (i.e., to the left of the telescope from the observer’s vantage) when the telescope is in the normal or direct position--thus, “face left.” When the telescope is inverted, the vertical circle is to the right of the telescope--thus, “face right.” Some texts refer to “face 1” as the equivalent to “face left” and “face 2” as the equivalent to “face right.” You may see other variations for designating telescope position. Some manufacturers use “V1” and “V2” with the “V” referring to the vertical circle to represent “face left” and “face right,” respectively. The important thing to remember is that these designations are used to refer to the position of the telescope and to confirm, in a full set of observations, that the telescope has been used in both positions.
When measuring horizontal and vertical angles, certain systematic errors in theodolites and total stations (and in the case of total stations, possibly errors in distances) may be eliminated by averaging observations made with the telescope in both positions. This is a good practice to follow even with modern electronic instruments--whether theodolite or total station--that have so-called collimation corrections, tilt compensation and other features. To ensure that errors are eliminated, be sure the number of measurements in one face are balanced by an equal number of measurements in the opposite face.
Q: I am sometimes required to follow procedures defined by the client or defined by the standards published by a particular surveying authority. When measuring angles, procedures often call for sighting the backsight target with 0° for one set then repeating the set with 90° on the backsight target. I think the reason for these procedures is to use a different part of the instrument’s circle so that averaging the separate observations obtains a more accurate result. However, I’ve noticed that those responsible for checking my work are satisfied if, when using an electronic instrument, I simply ‘zero set’ the circle for the first set and then key in 90° for the second set. I’m still using the same part(s) of the circle for each set, so have I accomplished anything?
A. You are absolutely correct about the reason for the procedures as written. In fact, what you describe is the process for measuring two sets of observations. If three sets were to be measured, the initial angles on the backsight would be 0°, 60° and 90°; if four, 0°, 45°, 90° and 135°. The increment in each case after the first measurement beginning with 0° is 180°/n where n is the number of sets to be measured. The primary errors you are trying to eliminate are circle graduation errors and errors in the angles due to the inability to perfectly locate the axis of rotation at the center of the graduated circle. This also presupposes that you are measuring each set in two faces (see above discussion) so that the second measurement actually uses yet a different part of the circle.
When these procedures were written, there were two primary types of angle-measuring instruments: direction and repeating. Both types varied the initial backsight angle setting as described above and did result in a number of observations using different parts of the circle. Then, when they were averaged, the likelihood of the errors from the causes described was minimized. While there are a few electronic theodolites and total stations that duplicate the motions and controls of the direction and repeating instruments, most today have no method for physically repositioning the circle unless the entire instrument is loosened on the tripod and then rotated the appropriate angle.
Unfortunately, many standards and specifications for the surveyors work have not been updated to truly reflect the instrument’s capabilities. You will need to carefully consider how your instrument works (often not described well enough in the user’s manual) before you can decide whether the procedures you currently follow enable the error reduction that the specification intended. Some instruments may use other techniques to accomplish this error reduction completely or partially and can even vary with the same manufacturer. Again, this will require research of your instrument’s particular hardware and software technology to determine its effectiveness at error reduction. Note that even when you are not using different parts of the circle, multiple measurements of angles is good practice for eliminating blunders.
GIAA is a trade association of manufacturers, suppliers and distribution partners encompassing the present and emerging technologies in surveying, GPS, engineering, construction, GIS/LIS and related fields. www.giaamerica.org .
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