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 electro-

optical 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.

Example 6.6

Δh = hB – hA = s.cosz + i – t

Terms:

S=Slope Distance, z=Vertical Angle.

i=The height of the instrument.

t=The height of the sight target.

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 = HB-HA= S.cos Z’ + HI - HT

Example 6.7

Example 6.8

Where HI and HT = Height measurement

of the instrument and the reflector of the

EDM device.

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 !