SURVEYING: ERROR SOURCES IN TOTAL STATION

SURVEYING: ERROR SOURCES IN TOTAL STATION

Total Station Error Sources

All theodolites measure angles with some degree of imperfection. These imperfections result from the fact that no mechanical device can be manufactured with zero error. In the past very specific measuring techniques were taught and employed by surveyors to compensate for minor mechanical imperfections in theodolites. With the advent of electronic theodolites, the mechanical errors still exist but are related to in a different way. One must now do more than memorize techniques that compensate for errors. One must clearly understand the concepts behind the techniques and the adjustments for errors that electronic theodolites now make. The following paragraphs provide the major sources of error when using a theodolite and also the particular method employed to compensate for that error.

a) Circle eccentricity

Circle eccentricity exists when the theoretical center of the mechanical axis of the theodolite does not coincide exactly with the center of the measuring circle. The amount of error corresponds to the degree of eccentricity and the part of the circle being read. When represented graphically circle eccentricity appears as a sine wave. Circle eccentricity in the horizontal circle can always be compensated for by measuring in both faces (opposite sides of the circle) and using the mean as a result. Vertical circle eccentricity cannot be compensated for in this manner since the circle moves with the telescope. More sophisticated techniques are required.

(1) Some theodolites are individually tested to determine the sine curve for the circle error in that particular instrument. Then a correction factor is stored in ROM that adds or subtracts from each angle reading so that a corrected measurement is displayed.

(2) Other instruments employ an angle-measuring system consisting of rotating glass circles that make a complete revolution for every angle measurement. These are scanned by fixed and moving light sensors. The glass circles are divided into equally spaced intervals which are diametrically scanned by the sensors. The amount of time it takes to input a reading into the processor is equal to one interval, thus only every alternate graduation is scanned. As a result, measurements are made and averaged for each circle measurement. This eliminates scale graduation and circle eccentricity error.

b) Horizontal collimation error

Horizontal collimation error exists when the optical axis of the theodolite is not exactly perpendicular to the telescope axis. To test for horizontal collimation error, point to a target in face one then point back to the same target in face two; the difference in horizontal circle readings should be 180 degrees. Horizontal collimation error can always be corrected for by meaning the face one and face two pointings of the instrument.

(1) Most electronic theodolites have a method to provide a field adjustment for horizontal collimation error. Again, the manual for each instrument provides detailed instruction on the use of this correction.

(2) In some instruments, the correction stored for horizontal collimation error can affect only measurements on one side of the circle at a time. Therefore when the telescope is passed through zenith (the other side of the circle is being read), the horizontal circle reading will change by twice the collimation error. These instruments are functioning exactly as designed when this happens.

(3) When prolonging a line with an electronic theodolite, the instrument operator should either turn a 180-degree angle or plunge the telescope and turn the horizontal tangent so that the horizontal circle reading is the same as it was before plunging the telescope.

c) Height of standards error:

In order for the telescope to plunge through a truly vertical plane the telescope axis must be perpendicular to the standing axis. As stated before there is no such thing as perfection in the physical world. All theodolites have a certain degree of error caused by imperfect positioning of the telescope axis. Generally, determination of this error should be accomplished by a qualified technician because horizontal collimation and height of standards errors interrelate and can magnify or offset one another. Horizontal collimation error is usually eliminated before checking for height of standards. Height of standards error is checked by pointing to a scale the same zenith angle above a 90-degree zenith in "face-one" and "face-two." The scales should read the same in face one as in face two.

d) Circle graduation error

In the past, circle graduation error was considered a major problem. For precise measurements surveyors advanced their circle on each successive set of angles so that circle graduation errors were “meaned out.” Current technology eliminates the problem of graduation errors. This is accomplished by photo-etching the graduations onto the glass circles and making a precise master circle and photographing it. An emulsion is then applied to the circle and a photo-reduced image of the master is projected onto the circle. The emulsion is removed and the glass circle has been etched with very precise graduations.

e) Vertical circle error

It is important to check the vertical circle indexing adjustment on surveying instruments on a routine basis. When direct and indirect zenith angles are measured to the same point, the sum of the two angles should equal 360°. Over time, the sum of these two angles may diverge from 360° and consequently cause errors in vertical angle measurements. While averaging the direct and indirect zenith angles easily eliminates this error, on many jobs it may not be cost effective to make two readings. Acceptable accuracy may still be maintained for many applications with only a direct reading; however, as long as the index error is kept to a minimum by periodically performing a vertical adjustment, such as TOPCON’s "Vertical Angle 0 Datum Adjustment." Most total stations are provided with some type of electronic adjustment to correct the vertical circle indexing error. This adjustment takes just a few seconds and will insure that you get good vertical angle readings with just one measurement. Consult the manufacturer’s manual for instructions on making this adjustment.

f) Pointing errors:

Pointing errors are due to both human ability to point the instrument and environmental conditions limiting clear vision of the observed target. The best way to minimize pointing errors is to repeat the observation several times and use the average as the result.

g) Uneven heating of the instrument.

Direct sunlight can heat one side of the instrument enough to cause small errors. For the highest accuracy, utilize an umbrella or pick a shaded spot for the instrument.

h) Vibrations

Avoid instrument locations that vibrate. Vibrations can cause the compensator to be unstable.

i) Collimation errors

When sighting points a single time (e.g., direct position only) for elevations, check the instrument regularly for collimation errors.

j) Vertical angles and elevations

When using total stations to measure precise elevations, the adjustment of the electronic tilt sensor and the reticule of the telescope becomes very important. An easy way to check the adjustment of these components is to set a baseline. A line close to the office with a  large difference in elevation will provide the best results. The baseline should be as long as the longest distance that will be measured to determine elevations with intermediate points at 100- to 200-ft intervals. Precise elevations of the points along the baseline should be measured by differential leveling. Set up the total station at one end of the baseline and measure the elevation of each point. Comparing the two sets of elevations provides a check on the accuracy and adjustment of the instrument. Accuracy requirements may dictate that more than one set of angles and distances is measured to each point. Some examples are distances over 600 feet, adverse weather conditions, and steep observations.

k) Atmospheric corrections

Meteorological data corrections to observed EDM slope distances may be significant over longer distances. Usually for most topographic surveying over short distances, nominal (estimated) temperature and pressure data is acceptable for input into the data collector. Instruments used to measure atmospheric temperature and pressure must be periodically calibrated. This would include psychrometers and barometers.

l) Optical plummet errors

The optical plummet or tribrachs must be periodically checked for misalignment. This would include total stations with laser plummets.

m) Adjustment of prism poles

When using prism poles, precautions should be taken to ensure accurate measurements. A common problem encountered when using prism poles is the adjustment of the leveling bubble. Bubbles can be examined by establishing a check station under a doorway in the office. First, mark a point on the top of the doorway. Using a plumb bob, establish a point under the point on the doorway. If possible, use a center punch to make a dent or hole in both the upper and lower marks. The prism pole can now be placed into the check station and easily adjusted.

n) Recording errors

The two most common errors are reading an angle incorrectly and/or entering incorrect information into the field book. Another common (and potentially disastrous) error is an incorrect instrument or rod height. Although electronic data collection has all but eliminated these errors, it is still possible for the surveyor to identify an object incorrectly, make a shot to the wrong spot, or input a bad target height (HR) or HI. For example, if the surveyor normally shoots a fire hydrant at the ground level, but for some reason shoots it on top of the operating nut, erroneous contours would result if the program recognized the fire hydrant as a ground shot and was not notified of this change in field procedure.

o) Angles

As a rule, a surveyor will turn a doubled angle for move-ahead, traverse points, property corners, or other objects that require greater accuracy. On the other hand, single angles are all that are required for topographic shots. Refer to the total station operating instructions for repeating angle methods where required.

p) Slope to grid and sea level EDM corrections

Slope distances will be reduced to horizontal distances in the data collector, and then reduced to a grid distance if a grid scale factor (or combined scale sea level factor) is input into the data collector. For most topographic survey applications involving short side shots, the grid scale factor is ignored (e.g., 1.000 is used). This would not be correct for control traverses covering larger distances. Scale factors can be obtained directly in CORPSCON.

q) EDM calibration

All EDM instruments should be periodically (at least annually) checked over a NGS Calibration Baseline or a baseline established by local state surveying societies.