Electronic Distance Measurements

1. DISTANCE MEASUREMENTS

2. DISTANCE MEASUREMENTS
• One of basic components surveys is the determination of the distance
between two points on the surface of the earth.
• Distance measurement methods differ in type instrument, in the
cost, in the length of the line as well as in the accuracy or quality.
DISTANCE MEASUREMENT
INDIRECT MEASUREMENT
Distance measurements
DIRECT MEASUREMENT
e.g. Length Measurements by
applying a tape.

3. Direct measurements
• Made by applying an instrument directly to the unknown quantity
and observing its value, usually by reading it directly from
graduated scales on the device.
• Determining the distance between two points by making a direct
measurement using a graduated tape or measuring an angle using a
Theodolite or total station instrument.

4. Indirect measurements
• Used when it is not possible or practical to make direct measurements.
• The quantity desired is determined from its mathematical relationship
to direct measurements. Surveyors may, for example, measure angles
and lengths of lines between points directly and use these
measurements to compute station coordinates.
• From these coordinate values, other distances and angles that were
not measured directly may be derived indirectly by computation.

5. Survey Equipment
•Electronic Distance Measuring Instruments
(i.e., Total stations and Theodolites)
•Measuring Tape

6. Electronic Distance
Measurements
• An electronic distance measuring instrument is a surveying
instrument for measuring distance electronically between
two points through electromagnetic waves.
• EDM instruments are highly reliable and convenient pieces of
surveying equipment and can be used to measure distances of
up to 100 kilometers.

8. Principles of Electronic Distance
Measurements (EDM)
• The underlying principle of modern EDM systems is based on the
speed of light. A light source is modulated and transmitted at a
very high frequency from the source (electronic total station) to a
distant reflector (prism).
• The reflector returns the light to the receiving optics of the
instrument and these light pulses are no longer in phase with the
outgoing light pulses because it has taken a certain amount of
time to travel from the instrument to the reflector and back again.
• These light pulses are converted to an electrical signal that is the
compared to a reference signal and a resulting phase delay can be
measured accurately.

9. Principles of Electronic Distance
Measurements (EDM)

10. Two Main Principles of Electronic Distance
Measurements (EDM)
Modern total stations use two (2) principles in analyzing the light
beam:
1. Phase difference measurements
2. Pulse measurements

11. Phase Difference Measurements
• In phase difference methods, electromagnetic waves are sent out
to a reflector in a form of sinusoidal waveform.
• The EM signal bounces off the surface of the reflective material
and returns to the instrument.
• The outgoing (emitted) and the returning (reflected) signals are
now out of phase.
• The instrument measures the amount by which the reflected
signal is out of phase with the emitted signal.
• Now the distance is measured by comparing the phase shift
between the transmitted and received signals

14. Pulse Methods (Time of Flight)
• In this method, the EDM generates many short infrared or
laser light pulses which are transmitted to target.
• These pulses reflect off the target and return to the
instrument, where electronics determine the total travel time
[Time Of Flight] for each pulse.
• The travel time [Time Of Flight] is then used to compute the
distance between the instrument and the target.
• The principle of this method depends on the concept that
distance traveled is the product of velocity and time.

15. Pulse Methods (Time of Flight)

16. Advantages of EDM Instruments
When compared to other methods, EDM has the following
advantages:
• The advantage of using EDM instruments lies on the speed
and accuracy in measurements.
• Several obstacles to chaining are automatically overcome
when these instruments are used.
• Less labor-intensive compared to tape/chain surveys.
• Less computations required and more precise
• Convenient and reliable

17. Advantages of EDM Instruments
When compared to other methods, EDM has the following
advantages:
• The advantage of using EDM instruments lies on the speed
and accuracy in measurements.
• Several obstacles to chaining are automatically overcome
when these instruments are used.
• Less labor-intensive compared to tape/chain surveys.
• Less computations required and more precise
• Convenient and reliable

18. Limitations of EDM Instruments
• Weather affects working of instrument
• Battery and electricity dependent
• Loss of data is an important consideration
• Typically, heavier than a transit or tape
• Initial cost is higher than Plane Table, Chain or Tacheometer
method

19. Natural Sources of Errors
• Variation of natural phenomena is also a possible source of
error.
• Atmospheric variations in temperature, pressure as well as
humidity. Microwave EDM instruments are more susceptible
to these.
• Multiple refraction of the signals, as the velocity of the EM
signal depends on the medium.

20. Instrumental Errors
• Calibration errors
• Errors shown by the reflectors.
Personal Errors
• Inaccuracy in initial setups of EDMs and the reflectors over
the preferred stations
• Instrument and reflector measurements going wrong
• Atmospheric pressures and temperature determination errors

21. Total Stations
• A total station is a type of digital theodolites that integrates
the angle-measuring tool with electronic distance
measurement functionality.
• All electronic total stations have an electronic optical distance
meter (EDM) and an electronic angle meter, so that the bar
codes of the horizontal and vertical circle scales can be read,
displaying the angles and distances values digitally.
• The horizontal distance, height difference and bearings are
calculated automatically.

22. THEODOLITES
• Theodolites or Transits are surveying
instruments designed to precisely measure
horizontal and vertical angles.
• They are used to establish straight and
curved lines.
• To establish Elevation when used as a level,
(when we set the vertical angle to 90°).

23. Theodolite