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mohd asnawi abd wahab

on 18 November 2014

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Transcript of EDM

Electronic Distance Measurement (EDM)
General Objective : To know and understand the basic principles and methods of using EDM/Total Station

Specific Objectives: At the end of the unit you should be
able to :-
Describe basic principles of distance measurement by electronic method.
Describe EDM/Total Station systems :
a. Microwave system
b. Electro optical system

EDM is an electronic distance measurement method that is capable of providing horizontal and vertical distance at high accuracies.
Details are measured either using an EDM mounted on a theodolite or a total station
This method of picking up details is closely linked to computer aided mapping
Data obversed are normally entered or downloaded to computers and processed using an engineering surveying software to produce the required plans

The wave is travelling along the x axis with a velocity of 299, 792.5 ± 0.4 km/s (in vacuum). The frequency of the wave is, the time taken for one complete wavelength.
lamda = c / ƒ
where; lamda = wavelength in meters
c = velocity in km/s
ƒ = frequency in hertz (one cycle per second)

Microwave system
A microwave system is a system of equipment used for microwave data transmission. The typical microwave system includes radios located high atop microwave towers, which are used for the transmission of microwave communications using line of sight microwave radio technology.
frequency of wave is 10Ghz (1 Ghz = 109 Hz)
Distance around 100km in sunny weather conditions
Range of maximum for EDM microwave is 25km – 30km
Accuracy is within +10mm / + 3mm per km
Tellumat, tellurometer, Geodimeter and Makometer

Electro optical system
The use of infrared EDM equipment is a simple and easy method in which most of the tools used to work - surveying.
The use of infrared EDM equipment Cause carrier wave is an infrared emitting diode arsenaid gallium (GaAs)
single prism limited to the range of 1 km but this can be added to the 2 or 3 km by using a reflector consisting of a sequence of 3 or 9 prism
Accuracy is within +10mm
Wild, Geodimeter, Sokia, topcon, leica and kern
Calibration of total stations
To maintain the high level of accuracy offered by modern total stations, there is now much more emphasis on monitoring instrumental errors, and with this in mind, some construction sites require all instruments to be checked on a regular basis using procedures outlined in the quality manuals.
For total stations, instrumental errors are measured and corrected using electronic calibration procedures that are carried out at any time and can be applied to the instrument on site

Specific Objectives: At the end of the unit you should be
able to :-
Demonstrate distance measurement by using electronic instruments.
Explain EDM/Total Station errors,effects accuracies and correction for measured slope distance.
The latest development of electronic distance measurement instrument.

Accuracies and correction for measured slope distance
Horizontal distance D = L cos = L sin z
Vertical distance = V = L sin = L cos z

DFT (Instrumental Distance Errors)
The accuracy of the distance measuring components of a total station can also decrease with constant use on site and with age. For this reason they should be tested and calibrated regularly. This can be done by carrying out a 3 peg test.
This error is of constant magnitude and does not depend on the length of the line measured. The method of calibration involves taking distance measurements along a 3 point baseline as shown above.

Total Stations can read and record horizontal and vertical angles together with slope distances.
The microprocessors in the Total Stations can perform a variety of mathematical operations: for example, averaging multiple angle measurements: averaging multiple distance measurements; determining X, Y, Z coordinates, remote object elevations (i.e., heights of sighted features), and distances between remote points; and making atmospheric and instrumental corrections.

EDM/Total Station errors & effects
Zero error :
Difference between the optical , mechanical and electrical centres of the EDM and reflecters (prism)
Scale error :
Caused by variation in the modulated frequency,f of the wave in the EDM
Cyclic error :
Caused by unwanted interference of the EDM’s signal
Atmosphere Effects: electromagnetic wave is affected when travel through atmosphere.therefore , correction are needed.Most modern EDM’s have facilities to correct for this effect

Motorised total stations

The latest generation of total stations have many of the features described in the previous section but are also fitted with servo-motors which control their horizontal and vertical movement. These are called motorised total stations
Robotic total stations

Although all motorised total stations can be used as conventional instruments might be, their full potential is realised when they are remote controlled and used as robotic total stations.
By providing remote control of the total station from the prism, these are systems that permit single-user operation for either mapping or setting out

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