Islamic University of Gaza Civil Engineering Department Surveying II ECIV 2332 By Belal Almassri
Chapter 6 Electronic Distance Measurement EDM - Introduction. - Electro-Optical Instrument. - Microwave Instrument. - Types of Mounts. - Operating Procedure. - Trigonometric Levelling. - Corrections and Examples.
Introduction Definition: A surveying instrument that utilizes an infrared or laser beam to measure the distance from the source point to a defined target point. Main Types: 1. Electro-Optical Instruments: Use the light and infrared waves. 2. Microwave Instruments: Use the radio waves.
Electro-optical Instruments Use the velocity of light waves to determine the distance between two points. Those instruments use both the visible lights (wavelength between 0.4-0.7) and the infrared lights (wavelength between 0.7-1.2). Infrared light much preferable because of the long wavelength. Source of light: Mercury lambs, Tungsten.
Electro-optical Principle S = Slope Distance. V = Velocity of Light. T = Travel Time.
Microwave Instruments Wave length λ = distance travelled during the period of one cycle. V = Velocity of emission F = Frequency in HZ
Disadvantages of Microwave Type Two operators are needed to measure a line. Sometimes there is difficulty to set up two units on both sides. The accuracy is affected by the atmosphere. Less accuracy than the electro-optical type.
Types of Mounts In General the EDM can be mounted alone or with another surveying tool, the most popular types as follows: Separate unit for (20-60 km). On top of standards of theodolite. On top of telescope of theodolite. Integral part of Total Station.
Retro Reflectors The reflectors used with the electrooptical type are called the retro-reflector using precise prisms which can be single or multiple. The max distance of measurement of EDM device depends on: The design, The quality and number of prisms, The atmospheric conditions.
Operating Procedure Set up the instrument over the survey station and record its height HI and set up the reflector on the other side. Point the instrument toward the reflector using the telescope. Adjust the pointing of tangents using the screws of the instrument. Read and record the Temperature, ATM pressure then press the correction button. Press measure and record the reading.
Corrections for zero centering The constant error appears in all distances measured by the combination of instrument and reflector. If AB is the measured distance between two points and so AC, CB The corrected AB = AB + c (correction) c = AB AC CB
Correction for frequency g = f / f f : measured frequency. f : theoretical frequency. AB (corrected) = g. AB (actual) + c Example 6.1 + 6.2
Trigonometric Levelling Short Line Def: It is the process of determining the height difference between two points using the measured distances and vertical angles.
Δh = h B h A = s.cosz + i t Terms: S=Slope Distance, z=vertical Angle. i=the height of the instrument. t=the height of the sight target. Example 6.6
In case of EDM mounted on top of the standards of the theodolite, the measured slope distance may not coincide with the line of sight in the angular measurement. So the following procedure is followed at that case: 1. Find the angle difference Δz. Δz = 180(HI-i+t-HT)sinz Π. S 2. Find the corrected angle Z =Z+ Δz 3. Find the height difference ΔH = H B -H A = S.cos Z + HI - HT
Where HI and HT = Height measurement of the instrument and the reflector of the EDM device. Example 6.7 Example 6.8
Extra Practice! EDM has slope distance AB of 561.276 m. EDM instrument is 1.820 m above station A, and the prism is 1.986 m above station B. The EDM is mounted on a theodolite whose optical center is 1.720 m above the station. The theodolite measured a vertical angle of +6º 21 38 to target on prism pole; the target is 1.810 m above station B. Compute both the horizontal distance AB and elevation of station B given an elevation at A of 186.275 m.
Solution!