Electronic Distance Measurement
Electronic Distance Measuring (EDM) equipment has developed since the first EDM apparatus was used in 1957. This instrument (called a Tellurometer) was able to measure distances up to 80 km, day or night. Although there is a very wide range of EDMs available, the basic principle of operation remains unchanged. The major improvements have arisen due to the development of integrated circuits (electronics). The resultant reduction in size of the instruments, ease of operation and relative decrease in cost has enabled almost every survey party to be equipped with some form of EDM.
The current range of EDM equipment available today varies from theodolite mounted units to EDM units fully integrated into the theodolite. These fully integrated units are termed total station instruments. Depending upon the type of EDM and ancillary equipment, the range of measurement may vary from a maximum of 500 metres to 100 kilometres.
• Wavelength of transmitted electromagnetic energy
• Maximum distance measured.
2. Microwave equipment, which transmits microwaves with wavelengths of 1.0 to 8.6 mm.
1. Short range group includes instruments with a maximum range of approximately 5 km. Most of these instruments are of the electro-optical type that employ infra-red light.
2. Medium range instruments have measuring capabilities extending to about 100 km and are either the electro-optical (using laser light) or microwave type. Although frequently used in precise geodetic work, they are also suitable for land and engineering surveys.
3. Long range instruments are available that can measure lines longer than 100 km, and are not generally used in engineering surveying work. Most operate by transmitting long radio waves or microwaves. They are primarily employed in oceanographic and hydrographic surveying, and navigation.
In the short range category, where engineering, cadastral and topographic surveying is most evident, technological advances have been the most significant. Because of smallness in size, portability and ease of use, these instruments are now used by practically all surveyors.
Because of the acceptance of EDM the variety of instruments has increased quite substantially, and competition between manufacturers is keen. It is in this category that the market is greatest because of the relatively low cost of the instruments and because more and more surveyors are accepting EDM as a tool that can return its purchase price in a very short time in certain applications.
Down time on most models of infra-red EDM is now a matter of days per year rather than weeks per year.
The range of instruments is wide and this is reflected in the variety of features on them as indicated below:
• Dial-in meteorological correction
• Dial-in instrument and prism constants
• Signal strength indicators of various types (audio, visual meter, visual light, etc)
• Tracking facility
• Slope distance reduction
• Vertical angle sensoring
• Service contracts
• Arithmetic mean of measurements made
• Horizontal angle reading
• Field data storage units (data recorder or electronic field book) where the electronic angles and distance are stored automatically in the field book, resulting in a saving of time and reduction of errors
• On board programs for most surveying applications. The most popular models in the short-range category seem to be those that can be mounted on any type of theodolite of a certain manufacturer; however, this is probably due to cost, as the combined function of an electronic theodolite and EDM (a total station) instruments tend to be more expensive.
Using `phase comparison', the EDM instrument uses a range of wavelengths to precisely determine the total length of the line.
1. Personal errors include misreading, improperly setting over stations, and incorrectly measuring meteorological factors, instrument heights and slope angles.
2. Instrumental errors for a carefully adjusted and precisely calibrated EDM should be extremely small.
Instrument errors are generally considered to be within a manufacturer's specified precision.
eg ± (5 mm + 5 ppm) short range EDM
All EDM instruments must be checked against known baselines at frequent intervals in order to verify their calibration.
3. Natural errors result primarily from atmospheric variations in temperature, pressure and humidity that modify the wavelength of electromagnetic energy. Most instruments provide tables for the correction for non-standard temperature and pressure values. These corrections are input into the EDM instrument at the time of measurement.
Classification of EDM Instruments
EDM instruments are commonly classified according to the following:• Wavelength of transmitted electromagnetic energy
• Maximum distance measured.
Wavelength of Transmitted Electromagnetic Energy
1. Electro-optical instruments that transmit either modulated laser or infra-red light, having wavelengths within or slightly beyond the visible region of the spectrum.2. Microwave equipment, which transmits microwaves with wavelengths of 1.0 to 8.6 mm.
Maximum Distance Measured
The boundaries of this classification system vary depending on the reference quoted; therefore, the following figures may be considered approximate.1. Short range group includes instruments with a maximum range of approximately 5 km. Most of these instruments are of the electro-optical type that employ infra-red light.
2. Medium range instruments have measuring capabilities extending to about 100 km and are either the electro-optical (using laser light) or microwave type. Although frequently used in precise geodetic work, they are also suitable for land and engineering surveys.
3. Long range instruments are available that can measure lines longer than 100 km, and are not generally used in engineering surveying work. Most operate by transmitting long radio waves or microwaves. They are primarily employed in oceanographic and hydrographic surveying, and navigation.
In the short range category, where engineering, cadastral and topographic surveying is most evident, technological advances have been the most significant. Because of smallness in size, portability and ease of use, these instruments are now used by practically all surveyors.
Because of the acceptance of EDM the variety of instruments has increased quite substantially, and competition between manufacturers is keen. It is in this category that the market is greatest because of the relatively low cost of the instruments and because more and more surveyors are accepting EDM as a tool that can return its purchase price in a very short time in certain applications.
Down time on most models of infra-red EDM is now a matter of days per year rather than weeks per year.
The range of instruments is wide and this is reflected in the variety of features on them as indicated below:
• Dial-in meteorological correction
• Dial-in instrument and prism constants
• Signal strength indicators of various types (audio, visual meter, visual light, etc)
• Tracking facility
• Slope distance reduction
• Vertical angle sensoring
• Service contracts
• Arithmetic mean of measurements made
• Horizontal angle reading
• Field data storage units (data recorder or electronic field book) where the electronic angles and distance are stored automatically in the field book, resulting in a saving of time and reduction of errors
• On board programs for most surveying applications. The most popular models in the short-range category seem to be those that can be mounted on any type of theodolite of a certain manufacturer; however, this is probably due to cost, as the combined function of an electronic theodolite and EDM (a total station) instruments tend to be more expensive.
Principles of EDM Operation
EDM instruments operate by the instrument transmitting electromagnetic energy to another instrument or reflector and then receiving the return signal. A common misconception is that the duration of travel is measured, and the distance thus deduced. In fact, a method of `phase comparison' is utilised to derive the measured distance.Using `phase comparison', the EDM instrument uses a range of wavelengths to precisely determine the total length of the line.
Reflectors.
The reflectors are made up of specifically designed prisms of glass, such that the signal from the EDM will be reflected directly back to the instrument.Errors Affecting EDM Operations
Sources of errors in EDM instrument work may be personal, instrumental, or natural.1. Personal errors include misreading, improperly setting over stations, and incorrectly measuring meteorological factors, instrument heights and slope angles.
2. Instrumental errors for a carefully adjusted and precisely calibrated EDM should be extremely small.
Instrument errors are generally considered to be within a manufacturer's specified precision.
eg ± (5 mm + 5 ppm) short range EDM
All EDM instruments must be checked against known baselines at frequent intervals in order to verify their calibration.
3. Natural errors result primarily from atmospheric variations in temperature, pressure and humidity that modify the wavelength of electromagnetic energy. Most instruments provide tables for the correction for non-standard temperature and pressure values. These corrections are input into the EDM instrument at the time of measurement.
0 comments:
Post a Comment