GIAA Mailbag: On Optical Total Stations
- Unexpected results
- Maintenance issues and user adjustments
- Storage and transportation
- Determination of product operating limits
- Repairability inquiries
- Survey instrumentation of all types
- Accessory and software inquiries
- Data transmission by data collectors and radios
- Data management and interoperability
Joseph V.R. Paiva will edit the column. However, each column will be the result of contributions from the technical staff of the GIAA member companies. In a further effort to reach users of survey instrumentation, GIAA will present a half-day workshop at the ACSM convention in Las Vegas next spring. Mark your calendars for an informative presentation, “How Things Work” on Sunday, March 18, 2001. You will hear from industry experts on the design concepts and other technical aspects of various components of optical and satellite surveying systems.
Q: The precision of my surveys is typically 1:50,000, sometimes as much as 1:100,000. Yet, when I survey a cut-off line between traverse points, or begin staking out from a pair of traverse points and close on another pair, the closure is often less than 1:10,000. Is my optical total station the culprit?
A: It is difficult to identify the source(s) of your errors without more information, preferably, through an on-site visit. However, your question brings to mind the single largest cause of errors of this type among surveyors who do “leap frog” traversing: optical plummet tribrachs. In responding to complaints of instrument accuracy we have seen optical plummets, in the extreme, being out of adjustment so as to cause positioning errors greater than 8 cm (>0.25 ft). We recommend that you have a competent repair shop check and adjust your plummets once a year—or learn to do it yourself. The adjustment of these plummets should be checked at least once a month, preferably weekly. Because they don’t rotate, as the alidade of your instrument does, you need a spacer (available at your instrument supplier) to check your plummet. To use it, set up a tripod inside your office, mount any tribrach on it and put a spacer inside the tribrach. Now place the tribrach to be adjusted upside-down on the spacer. Look through the eyepiece and adjust the leveling screws to align the cross hairs on a sharply defined point on the ceiling. (Note that your tribrachs do not need to be leveled.) Now rotate the upper tribrach 180 and check the position of the cross hairs. If they have moved, the apparent movement is twice the error in the optical plummet. The GIAA members commend you on the method of making a redundant measurement to check the accuracy of your surveying system.
Q: When I set up my optical total station in the cold, after it has been sitting in my warm truck, I sometimes hear pops and pings. Do these sounds indicate damage?
A: It is very unlikely damage is being caused. Surveying instruments are designed for use outdoors. However, you may be “damaging” your surveys if you don’t allow enough time for your instrument to reach thermal equilibrium. A rule of thumb for acclimating your instruments to the ambient temperature is one to two minutes per degree Fahrenheit. If you work in Celsius, use two to four minutes per degree. You can get a jump-start on acclimatization by carrying your instrument in a section of your vehicle that exposes it to ambient temperatures, however, make sure to protect it from shock and moisture. The thermal shock can cause temporary strains and stresses in the instrument’s housings. Expansion or contraction of dissimilar materials at varying rates aggravate this. It is also possible to have instability in your system electronics during the period of acclimatization. Given an adequate period of adjustment, these conditions will settle and the equipment will perform as specified. Condensation may also be a problem, especially when going from warm to cool. However, the inconvenience of the condensation that may form on the outside of the telescope is not the only effect. Many optical total stations use optical components to measure horizontal and vertical angles. Thus, using your unacclimated instrument may compromise accuracy. Thermal stability is important to assure that the angles being measured are accurate. Thermal instability can have especially serious consequences on horizontal angle measurements, particularly when the line of sight is not horizontal, and on vertical angle measurements regardless of the inclination of the telescope. When you bring your instrument back to the office, be gentle with it. Let it cool down or warm up with the instrument case lid ajar so that dry air can circulate freely around it. Should your work require constant exposure to either extremely cold or hot conditions, contact the equipment manufacturer. In many cases there are special modifications, enhancements or accessories that can be utilized to insure successful operation.
POB would like to acknowledge the contributions of Brad Rozelle of Trimble for this month’s column. Please E-mail your questions to Diana Brown at firstname.lastname@example.org. Although we can’t print all questions, we will attempt to cover as many as possible. You also may ask your questions or continue the discussion at POB’s industry bulletin board, www.rpls.com.