- Sources of Errors in Total Station:
Total station is a device used for surveying, but like any device, it has its own sources of error which can affect the surveying report. These errors can be mechanical in nature or due to electronic components used in the total station. Understanding the sources of errors and the techniques used to compensate for them is important in order to get accurate measurements. - Mechanical Errors in Total Station:
The mechanical errors in total station arise due to imperfections in the measuring device. These imperfections are caused by the fact that no mechanical device can be manufactured with zero error. In the past, surveyors used specific measuring techniques to compensate for the minor imperfections in theodolites. - Electronic Errors in Total Station:
With the advent of electronic theodolites, the mechanical errors still exist but are related to in a different way. Electronic errors in total station can be due to faulty electronic components used in the station. - Understanding the Techniques for Error Compensation:
In order to get accurate measurements, it is important to understand the concepts behind the techniques and the adjustments for errors that electronic theodolites now make. Surveyors must learn the techniques used to compensate for the errors in order to get reliable measurements.
Error Sources in Total Station in Surveying
Circle
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What is Circle Eccentricity?
Circle eccentricity exists when the center of the mechanical axis of the theodolite does not align perfectly with the center of the measuring circle. This creates an error which is represented as a sine wave graphically, and is related to the part of the circle being read. To compensate for errors in the horizontal circle, it is necessary to average the readings taken from each face. However, this is not possible for errors in the vertical circle as the circle moves with the telescope.
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Methods of Compensating for Circle Eccentricity
To compensate for circle eccentricity, there are two methods that can be employed. Firstly, some theodolites are tested to determine the exact shape of the circle error and a correction factor is stored in the instrument’s ROM to account for this. Alternatively, angle-measuring systems incorporating rotating glass circles and fixed and moving light sensors are used. This system scans every alternate graduation, allowing measurements to be taken and averaged for each circle measurement, thereby eliminating scale graduation and circle eccentricity errors.
Horizontal Collimation Error in Total Station
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Understanding Horizontal Collimation Error
Horizontal collimation error occurs when the optical axis of the theodolite is not in exact perpendicular alignment with the telescope axis. To accurately test for this type of error, the same target should be pointed to in both faces one and two; the difference in horizontal circle readings should be 180 degrees. This can be corrected for by taking the mean of both the face one and face two pointings.
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Horizontal Collimation Error Adjustment on Electronic Theodolites
Most electronic theodolites provide a field adjustment for horizontal collimation error. Refer to the manual of the instrument to understand how to use this feature. Additionally, some instruments may have a correction stored for horizontal collimation error, however this only affects one side of the circle at a time, and so when the telescope passes through zenith, the reading will change by double the collimation error.
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Prolonging a Line with an Electronic Theodolite
When prolonging a line with an electronic theodolite, the operator should either turn a 180-degree angle or plunge the telescope and turn the horizontal tangent, so the horizontal circle reading remains the same as it was before plunging the telescope.
Height of Standards Error in Total Station
How to Plunge Through a Truly Vertical Plane
In order for a telescope to plunge through a truly vertical plane, the telescope axis must be perpendicular to the standing axis. Although perfection is impossible in the physical world, due to the imperfect positioning of the telescope axis, achieving the closest result possible is possible through proper calibration. This should be completed by a qualified technician, and involves both horizontal collimation and height of standards errors. Horizontal collimation should be checked and eliminated before height of standards is evaluated. Height of standards is checked by pointing the telescope to a scale at the same zenith angle above a 90-degree zenith in “face-one” and “face-two.” The scales should read the same in both positions.
Circle Graduation Error in Total Station
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Circle Graduation Error: A Major Problem in the Past
In the past, circle graduation error was a major problem for surveyors who needed precise measurements. To address this issue, surveyors advanced their circles on each set of angles, hoping to “mean out” any errors.
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Modern Technology Eliminates Circle Graduation Error
Current technology has eliminated the issue of circle graduation errors. This is achieved through a process of photo-etching the graduations onto the glass circles, creating a precise master circle, and applying an emulsion to the circle. This allows a photo-reduced image of the master to be projected onto the circle, resulting in etched graduations that are very precise.
Vertical Circle Error in Total Station
- What is Vertical Circle Indexing Adjustment?
Vertical circle indexing adjustment is the process of checking the accuracy of surveying instruments on a regular basis to ensure that they are providing accurate measurements. This adjustment is necessary to ensure that the sum of direct and indirect zenith angles is equal to 360°. Without this adjustment, errors in vertical angle readings may occur.
- Why is Vertical Circle Indexing Adjustment Important?
Vertical circle indexing adjustment is essential in order to maintain the accuracy of surveying instruments and to ensure that vertical angle measurements are accurate. Without this adjustment, errors may accumulate over time, resulting in incorrect readings.
- How is Vertical Circle Indexing Adjustment Performed?
Most modern total stations are equipped with an electronic adjustment to correct the vertical circle indexing error. This adjustment typically takes only a few seconds and can be done according to the instructions in the manufacturer’s manual. Additionally, averaging the direct and indirect zenith angles can help to reduce errors in vertical angle measurements.
Pointing Errors in Total Station
What Are Pointing Errors?
Pointing errors refer to the inaccuracy of an instrument when used to point to a specific target. This can be caused by human error or environmental conditions that impede clear vision.
How to Minimize Pointing Errors
The best way to reduce pointing errors is to take multiple readings of the same target and use the average as the result. This helps to reduce errors and increase accuracy.
Uneven Heating of the Instrument
Avoid Direct Sunlight for Highest Accuracy
Direct sunlight can put an instrument at risk of small errors due to the heat it can generate on one side of the instrument. To ensure the highest accuracy, it is recommended to use an umbrella or pick a shaded spot to place the instrument.
Vibrations
Avoiding Instrument Locations that Vibrate
It is important to avoid instrument locations that vibrate, as vibrations can lead to the compensator becoming unstable. Vibrations can cause the compensator to become less reliable and accurate, which can have significant impacts on the data collected. In order to ensure the accuracy and reliability of the data collected, it is best to avoid instrument locations that vibrate and opt for those that are more stable.
Collimation Errors
Sighting Points for Elevations: Check Instrument for Collimation Errors
When performing a single sighting of points for elevations, it is important to regularly check the instrument for collimation errors. This is to ensure that the instrument is properly aligned and that the readings are accurate. Collimation errors can occur if the instrument is not properly calibrated and can lead to inaccurate readings. By regularly checking for collimation errors, the accuracy of the readings can be verified and any errors can be corrected.
Vertical Angles and Elevations
HOW TO CHECK TOTAL STATION ADJUSTMENT
Checking the adjustment of the electronic tilt sensor and the reticule of the telescope on a total station is an important step in measuring precise elevations. One way to do this is to set up a baseline near the office, with a large difference in elevation, that is as long as the longest distance to be measured. Intermediate points should be set up at intervals of 100-200 feet. This baseline should then be measured using differential leveling, where the elevation of each point is measured using the total station. Comparing the two sets of elevations will provide a check of the accuracy and adjustment of the instrument. Accuracy requirements may require more than one set of angles and distances to be measured for each point, depending on factors such as the distance, weather conditions, and steepness of the measurement.
Atmospheric Corrections in Total Station
Meteorological Data Corrections for EDM Slope Distances
When it comes to topographic surveying, meteorological data corrections to measured EDM (electronic distance measurement) slope distances may be necessary. Depending on the distance of the survey, accurate atmospheric temperature and pressure data must be taken into account. In many cases, nominal (estimated) temperature and pressure data can be used when surveying shorter distances.
Instrument Calibration for Atmospheric Measurement
In order to measure atmospheric temperature and pressure accurately, instruments like psychrometers and barometers must be regularly calibrated. This ensures that the data collected is accurate and reliable. Without proper calibration, the survey results may be impacted and unreliable.
Optical Plummet Errors
Optical Plummets and Tribrachs: Maintaining Accuracy
Optical plummets and tribrachs are essential pieces of equipment for surveying, engineering, and construction projects. Without accurate measurements, projects may fail or be completed with costly mistakes. To ensure accuracy and success, it is important to periodically check optical plummets and tribrachs for any misalignment. This includes checking laser plummets on total stations. By regularly monitoring the alignment of these tools, surveying professionals can ensure that their measurements are accurate and that their projects are successful.
Adjustment of Prism Poles
Using Prism Poles to Achieve Accurate Measurements
When using prism poles to take measurements, it is important to take necessary precautions to ensure accuracy. One of the most common issues that can arise is incorrect adjustment of the leveling bubble. To prevent this, a check station should be established under a doorway in the office. Begin by marking a point on the top of the doorway, then use a plumb bob to establish a point under the point on the doorway. A center punch can be used to make a dent or hole in both the upper and lower marks. By using this check station, the prism pole can be easily adjusted.
Recording Errors
Common Mistakes in Surveying
Surveying involves the collection of data in order to accurately map an area. However, mistakes can occur due to incorrect readings or incorrect data entry into field books. Two of the most common errors are reading an angle incorrectly and entering incorrect information into the field book.
Incorrect Instrument or Rod Height
Another common, yet potentially disastrous, mistake is incorrect instrument or rod height. This can lead to erroneous contours if the program is not notified of any changes in field procedure. For example, if a fire hydrant is usually shot at ground level, but is shot at the operating nut for some reason, the program needs to be informed of this change in order to avoid errors.
Advantages of Electronic Data Collection
The introduction of electronic data collection has virtually eliminated these types of errors. This has made the surveying process more accurate and efficient, resulting in fewer mistakes and more accurate maps.
Angles
A Rule for Using Doubled Angles
Surveyors often use doubled angles for greater accuracy when establishing traverse points, property corners, and other objects. Doubled angles are also used in move-ahead surveys, which require greater accuracy than topographic shots.
How to Use Single and Doubled Angles
When surveying, single angles are normally sufficient for topographic shots while doubled angles are used for points requiring greater accuracy. Different methods of repeating angles may be used depending on the instructions provided in the Total Station operating manual.
Slope to Grid and Sea Level EDM Corrections
Slope Distance Reduction
In order to accurately measure distances in topographic surveys, it is necessary to reduce slope distances to horizontal distances. This is done using a data collector, which accepts a grid scale factor (or combined scale sea level factor) to convert the horizontal distances into grid distances. For most short-range surveying applications, the grid scale factor is ignored and replaced with 1.000. However, when surveying longer distances, a scale factor can be obtained directly from CORPSCON.
Accurate Measurement
Accurate measurements are essential when conducting topographic surveys. By reducing slope distances to horizontal distances, and then converting them to grid distances using a scale factor, surveyors can ensure that their measurements are as accurate as possible. This is especially important when surveying long distances, as even small errors can add up over a larger area. By using CORPSCON to obtain scale factors, surveyors can ensure that their measurements are as precise as possible.
EDM Calibration
Periodic Checks of EDM Instruments
Periodic maintenance of EDM instruments is essential for ensuring accurate and reliable measurements. NGS Calibration Baseline or a baseline established by local state surveying societies should be used to verify the accuracy of the instrument at least once a year. This will help ensure that the instrument is functioning properly and producing accurate results. Additionally, any discrepancies that may arise during the checking process can be identified and addressed quickly. By regularly verifying the accuracy of EDM instruments, surveyors can be sure that their measurements are reliable and consistent.