A total station is a gadget that is used to replace a total station in surveying. Total stations, like any other equipment, contain certain sources of mistake that can alter the surveying report. These sources of total station errors are discussed. With some degree of imperfection, all theodolites measure angles. These flaws arise from the fact that no mechanical equipment can be constructed without making mistakes.
In the past, surveyors were trained and used extremely particular measuring techniques to adjust for tiny mechanical defects in theodolites. Mechanical mistakes still occur with the advent of electronic theodolites, but they are dealt with in a different fashion. More than memorising ways to adjust for faults is now required. The concepts underlying the procedures must be fully understood.
The principal sources of error while using a theodolite are discussed in the following paragraphs, as well as the specific approach used to adjust for that inaccuracy.
Total Station Error Sources in the Surveying circle
Circle eccentricity occurs when the theoretical centre of the theodolite’s mechanical axis does not exactly match with the centre of the measurement circle. The degree of eccentricity and the part of the circle being read determine the amount of inaccuracy. Circle eccentricity is depicted graphically as a sine wave. In the horizontal circle, eccentricity can always be corrected for by measuring both faces (opposite sides of the circle) and taking the mean. Because the circle moves with the telescope, the eccentricity of the vertical circle cannot be adjusted in this way. Techniques that are more advanced are necessary.
Individually tested theodolites are used to determine the sine curve for the circle error in that instrument. Then, in the ROM, a correction factor is stored that adds or subtracts from each angle reading to produce a corrected measurement. Other instruments use an angle-measuring system that consists of spinning glass rings that complete a full rotation for each angle measurement. Light sensors, both fixed and moving, scan these. The glass circles are separated into evenly spaced spaces that the sensors scan diametrically. Because one interval is equivalent to the time it takes to feed a reading into the processor, only every alternate graduation is scanned. As a result, for each circle measurement, measurements are taken and averaged.
Total Station Horizontal Collimation Error
When the theodolite’s optical axis is not exactly perpendicular to the telescope axis, horizontal collimation error occurs. Point to a target in face one, then back to the same object in face two to check for horizontal collimation error; the difference in horizontal circle readings should be 180 degrees. Horizontal collimation inaccuracy can always be addressed by referring to the instrument’s face one and face two pointings. Most electronic theodolites have a means for correcting horizontal collimation inaccuracy in the field. Again, complete instructions on how to employ this correction can be found in the manual for each instrument.
The horizontal collimation error correction stored in some instruments can only effect measurements on one side of the circle at a time. As a result, the horizontal circle reading will shift by twice the collimation error when the telescope is passed through zenith (the other side of the circle is read). When this happens, the instruments are performing just as they should. When using an electronic theodolite to extend a line, the instrument operator should either turn the telescope 180 degrees or plunge the telescope and change the horizontal tangent so that the horizontal circle reading is the same as before plunging the telescope. Total Station Height of Standards Error The telescope must plummet through a really vertical plane in order to do this.
Total Station Error in Circle Graduation
Because the telescope axis is not perfectly aligned, all theodolites have some degree of inaccuracy. Because horizontal collimation and height of standards flaws are intertwined and can exacerbate or offset one another, this error should be determined by a competent technician. Before verifying for standard height, horizontal collimation error is normally eliminated. In “face-one” and “face-two,” the height of standards inaccuracy is checked by pointing to a scale with the same zenith angle above a 90-degree zenith. In both faces one and two, the scales should read the same. In the past, circle graduation inaccuracy was regarded as a serious issue. Surveyors advanced their circle on each subsequent set of angles for exact measurements.
This is done by photo-etching the graduations onto the glass circles and then shooting an exact master circle. The circle is then coated with emulsion, and a photo-reduced image of the master is projected onto it. The emulsion has been removed, and the graduations on the glass circle have been engraved with extreme precision. Total Station Vertical Circle Error On a regular basis, surveying instruments should have their vertical circle indexing adjustment checked. When the sum of the direct and indirect zenith angles is measured at the same point, it should equal 360°. The sum of these two angles may deviate from 360° over time, resulting in vertical angle measurement inaccuracies. When combining the direct and indirect effects.
Heating of the Instrument Isn’t Even
To address the vertical circle indexing inaccuracy, most total stations have some form of electronic correction. It only takes a few seconds to make this adjustment, and it ensures that you get accurate vertical angle measurements with just one measurement. Instructions for making this modification can be found in the manufacturer’s manual.Total Station Pointing Errors Both human capacity to point the instrument and environmental circumstances restricting good vision of the detected target contribute to pointing mistakes. The most effective technique to reduce pointing errors is to repeat the observation numerous times and take the average. Heating of the Instrument Isn’t Even Direct sunlight might create slight inaccuracies by heating one side of the instrument. Use an umbrella or a pick for the best precision.
When utilising total stations to measure precise altitudes, it’s critical to adjust the electronic tilt sensor and the telescope’s reticule. Setting a baseline is a simple technique to assess the adjustment of these components. The best results will come from a line that is close to the workplace and has a considerable variation in elevation. The baseline should be as long as the longest distance to be measured in order to estimate heights, with intermediate points every 100 to 200 feet. Differential levelling should be used to estimate precise heights of sites along the baseline. Place the total station at one end of the baseline and take elevation measurements at each location. The correctness of the two sets of heights can be checked by comparing them.
Total Station Atmospheric Corrections
Over longer distances, meteorological data corrections to measured EDM slope distances may be significant. For most short-distance topographic surveying, nominal (estimated) temperature and pressure values are suitable for input into the data collector. Instruments that measure the temperature and pressure of the atmosphere must be calibrated on a regular basis. Psychrometers and barometers are examples of this.Errors in Optical Plummet The optical plummet or tribrachs must be examined for misalignment on a regular basis. Total stations with laser plummets fall into this category. Precautions should be taken when utilising prism poles to ensure precise measurements. The adjustment of the levelling bubble is a common challenge when utilising prism poles. Establishing a check station under a doorway in the office might be used to analyse bubbles.
Vertical collimation error, Centering error, Horizontal Collimation error, Eccentricity error.
Common sources of error include instrumental, environmental, procedural, and human.
The worker cannot able to observe at the time of working because it complicates his work.
Also Check
Core Sampling and Testing of Concrete and Factors Affecting Strength