Errors in Electronic Distance
Measurements
Hamit Erdoğan
Civil Engineering Faculty
Geodesy and Photogrametry
Engineering
010000644
English 201
Esra Temel
29.12.04
Errors in Electronic Distance Measurement
Thesis: Although EDM is one of the most accurate methods of surveying, the distance
measurement obtained in the field have some errors, not only personal errors
but also instrumental and natural errors affect the electronic distance
measurements.
I. Personal errors
A. Blunders
1. Centering errors
2. Improper pointing
3. Improper voltage
4. Improper meteorological data
5. Instrument leveling error
B. Limitations of human senses
1. Reading errors
2. Observing wrong point.
II. Instrumental errors
A. Additive constant
B. Short periodic errors
1. Electrical or optical crosstalk errors
2. Analogue phase measurement errors
3. Multipath errors
C. Scale errors
III. Natural errors
A. Refraction index
B. Pressure
C. Temperature
D. Humidity
Since the existence of mankind, human needed to know their environment. In the course of time human understand that the best way to know the world is measuring the world. After that they started to measure the world. In early ages they developed some techniques which were very simple such as, using wooden sticks, ropes and chains etc. to measure their buildings, walls and roads. After years passed they developed steel tape which was one of the important developments but it was not sufficient for surveying. Because they could not measure the long distances, it was not easy and quick, also it was not very accurate. And finally they developed electronic distance measurement techniqueduring the World War II. It was a big step for surveying and mankind. Measuring distance became easier, faster, and the most important is, it became more accurate. Although electronic distance measurement (EDM) is one of the most accurate method of surveying, distance measurement obtained in the field have some errors, not only personalerrors but also instrumental and natural errors affect the electronic distance measurements.
First of all, personal errors are mostly coincidental.They can be reduced by training inattention and by using good quality instruments.Anderson asserts that being so rough; blunders are not exactly errors that are different from the other kinds of errors(Anderson,1985 p19).Furthermore,as mentioned inSurveying Measurements, when crew makes observations, blunders can not be estimated at the beginning or during the operation. But blunders are so obvious that an operator can easily understand there is a mistake which is done by the crew during the observation(Surveying Measurements,n.d p.2).Moreover, if the vertical axis of electronic distance meter or reflector is not fit into the check point, the centering error will occur.(Sources of measurement,2002, p4-5).For
receiving width amount of signalelectronic distance meters have electronic pointing and also it should be done correctly. After the fine motion screws are setup, electronic pointing provides to get maximum amount of signal. Receiving powerful signal supplies making correct measurements. Because of electronic distance measurement phases that are not homogeneous in short ranges, electronic pointing becomes more important and effective for EDM(Sources of measurement,2002, p. 4-3).In addition, Dracup emphasizes that, batteries of the instrumentsmust be completely charged to work with perfect performance (Dracup,1973, p.47). He also states that, improper readings of meteorological data occur, if good quality thermometers, barometers, etc. is not used(Dracup, 1973, p. 47).Due to the plate level bubble of the instrument is to susceptible, paying attentionbecomes important while leveling the instruments(Sources of measurement,2002, p. 4-4). In conclusion of this paragraph, blunders are not complicated as other errors and they can be easily realized and eliminated by repeating the measurements. Another point is, training of the measuring team is very important to decreasing the blunders.
Also limitations of the human senses cause errors in electronic distance measurements.According to the Anderson, generally incorrect readings of values, numbers, scales etc. cause errors because of inattentiveness of the operator (Anderson, 1985, p.19).Anderson states in the same page that, because of carelessness, observing the wrong point causes errors (Anderson, 1985, p. 19). Most of the personal errors appear because of these reasons. Because of this paying attention and being careful must be the most important features of a surveyor.
Secondly, not only in the pulse measurement but also in the phase measurement systems whole EDM instruments complain about lots of instrumental errors. The
electrical and optical representation of the instrument or production and structure of the instrument cause these errors. The producers of the instruments reduce most of these errors and determine them as a report to the users. The rest of the errors must be adjusted by the operator of the instrument and the operator mustalso know the reason of the error (Rüeger, 1996, p. 174). Additive constant is one of the most important instrumental errors. As Rüeger pointed out 1996:
Because the virtual electro-optical origin or zero of an EDM instrument is usually not located on the vertical axis of the instrument, a small correction has to be added to all distance measurements to refer the distance to the instrument’s vertical axis. This correction is usually called the additive constant compensates for electrical cable and component delays as well as optical path lengths (p. 174).
Furthermore, according to the Rüeger, while EDM instruments that depend on phase measuring make calculation, the periodic errors may reveal up to the size of the unit lengthU. These errors occur in small values of U or its harmonics. In phase measuring principle, systematic errors and optical or electrical crosstalk may lead to periodic errors. On the other hand, the crosstalk is not a matter for the instrument which use pulse, because the pulses which are not sent and taken diverge in time (Rüeger, 1996, p. 175). Moreover, on the same page Rüeger mentioned that, “Short Periodic errors may be caused by electrical coupling between the reference signal and the measurement signal and by optical crosstalk between the transmitter and receiver optics in electro-optical distance meters” (Rüeger, 1996, p. 175). Besides, Rüeger claims that, initial instruments which use analogue phase measuring systems are usually equipped with phase changing resolvers. Wavelengths that are the same length of the half of the unit length of EDM instruments show sinusoidal errors or secondary small systematic errors. Analogue phase
measuring systems are the reason of all systematic errors whose abundance is stable as a result they are not contingent to the remoteness (Rüeger, 1996, p. 177-178).In addition, according to Rüeger, transmitted signals which the instrument and the reflector, are the reason of the initial and greater sequence of the short periodic errors. Some parts of the EDM instrument can reflect the signal which is received from the reflector again and again. Because of this repetition the intensity of the signal is decreased. Smooth surfaces in the environment can also affect the reflection of the signal. If the distance gets longer the multipath errors will be smaller (Rüeger, 1996, p. 178).It is very important for getting the accurate data, because distances are measured by calculating the period of the signals that travel from instrument and reflector and if multipath errors occur,period of the traveling signal will change so, the measurements become inaccurate. In addition, as mentioned in“Calibration”, the oscillator and transmitting, receiving diodes are the main reasons of the scale errors in EDM instruments. Also other environmental conditions like velocity of light, humidity, temperature, pressure, and geometric reduction are the reasons of the big amount of the scale errors. As a result, instrumental errors constitute the most important part of errors. They are complicated than other errors and it is hard to eliminate these errors without using mathematical calculations.
Thirdly, natural errors are the last sources of errors that affect the electronic distance measurements. According to the “Extending Control”, getting improper measures of the refractive index makes the natural errors to pass over the instrumental errors. During the instrument calculations the proper measures can be used instead of acceptable measures which are distinct from proper measures. The water vapor is the most effective condition for refractive index because of the dispersion the gases in the atmosphere, temperature, pressure and the signal frequency. To get proper true mean
values, atmospheric conditions should be measured around the EDM instruments and reflectors(Extending Control, n.d, p. 3-4).Furthermore, Rüeger claims that, “An error of 1 mb affects distances by only 0.3 ppm. Depending on the length of the EDM lines and the resolution of the distance meter used, good quality aneroid barometers are generally sufficient.”(Rüeger, 1996, p. 60) Besides, Dracup thinks that, measuring of the temperature should be done 6 to 10 feet high from the surface to decrease ground radiation (Dracup, 1973, p.54).Dracup also states that, the effect of the humidity to the microwave instruments is much more than the effect to the lightwave instruments (Dracup, 1973, p. 47). We can easily understand that meteorological conditions are also important for distance measurements. We can not neglect these errors and we must eliminate them like other sources of errors.
In conclusion, distance measurements are necessary for surveying. It is impossible to make their job without measuring distances, for the surveyors. Nowadays distance measurements are easier owing to development of the EDM. Due to being accurate, fast and useful EDM instruments become widespread around the world. In spite of being useful and accurate EDM has also some errors like the other surveying methods. One of the most important missions of a surveyor is eliminating these errors and getting the most accurate data. To eliminate these errors a surveyor must know the sources of errors that are; personal errors, instrumental errors and natural errors. This is the aim of the surveying and surveyors should be well educated about these errors.
References
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