Measurement of distance is accomplished with a modulated microwave or infrared
carrier signal, generated by a small solid-state emitter within the instrument's optical path,
and bounced off of the object to be measured.
The modulation pattern in the returning signal is read and interpreted by the onboard
computer in the EDM.
The distance is determined by emitting and receiving multiple frequencies, and
determining the integer number of wavelengths to the target for each frequency.
In EDM the beam of light is the carrier and which is reflected back from mirror located
at the other end. Such instrument are less expensive because one active instrument and
battery are only needed at one end and instrument at other end is simply a reflecting
mirror centered over ground centre mark
Infrared Wave Instruments
•These instruments measure distances by using amplitude
modulated infrared waves. (prisms mounted on target are
used to reflect the waves.)
•These instruments are light and economical and can be
mounted on theodolites for angular measurements.
•The range of such an instrument will be 3 km and the
accuracy achieved is ±10 mm.
Light Wave Instruments
•These are the instruments which measures distances
based on propagation of modulated light waves.
•The accuracy of such an instrument varies from 0.5 to
5 mm / km distance and has a range of nearly 3 km.
•These instruments make use of high frequency radio
•Thee instruments were invented as early as 1950 in
South Africa by Dr. T.L. Wadley.
•The range of these instruments is up to 100 km and
can be used both during day and might.
Hand held EDM
Can be used with accuracy
of 10mm or so
Measurement of moving
A total station integrates the functions of a theodolite for measuring
angles, an EDM for measuring distances, digital data and a data recorder.
All total stations have similar constructional features regardless of
technology, and all perform basically the same functions.
Total solution for surveying work,
Most accurate and user friendly,
Gives position of a point (x, y and z) w. r. t. known point
Measures distance and angles and displays coordinates,
EDM is fitted inside the telescope,
On board memory to store data
Compatibility with computers
Measures distance and angles and displays coordinates
Auto level compensator is available
Can work in lesser visibility also
Can be used for curve layout after feeding data.
Total Stations can be used for:
• General purpose angle
• General purpose distance
• Slope measurement
• Contour and detail mapping
• Setting out and construction work
Distance measured with laser up to 2
Distance measured with infrared rays
up to 4 KM.( with
Capable of storing up to 20,000
Advantages of Total Station over Conventional
Traditional survey methods are laborious and time consuming
Fully automatic electronic measurement
Digital display of staff reading and distance
Data storage in instrument possible (5000 points )
Direct transfer to personal computer of data stored in instruments
Online operation through integrated interface to computer
Total stations are dependent on batteries and electronics. The LCD
screen does not work well when it is cold.
Battery life is also short, batteries and electronics both do not work well
Loss of data is an important consideration.
What is GPS?
GPS is a space-based, radio-navigation system that provides worldwide, all-weather,
three-dimensional position, velocity, navigation, and time data to both civilian and
Potential uses for GPS within the highway community are diverse and range from
providing traveler information to mapping (GPS technology can be integrated easily
with Geographic Information Systems).
How does it work?
GPS can provide a very accurate digital map of the highway infrastructure.
The technology operates on the principle of triangulation—if the difference from an observer
to three known points can be measured, the position of the observer can be calculated.
The system includes at least 24 satellites in orbit 19,320 kilometers (12,000 miles) above the
earth and inclined at 55°. These satellites continuously broadcast their position, a timing
signal, and other information.
By combining the measurements from four different satellites, users with receivers can
determine their 3-dimensional position, currently within 4–20 meters (13–66 feet).
Surveying and mapping was one of the first commercial adaptations of GPS, as it
provides a latitude and longitude position directly without the need to measure angles
and distances between points.
However, it hasn’t entirely replaced surveying field instruments such as the
theodolite, Electronic Distance Meter, or the more modern Total Station, due to the
cost of the technology and the need for GPS to be able to ‘see’ the satellites therefore
restricting its use near trees and tall buildings.
GPS is especially useful in surveying coasts and waterways, where there are few land-
based reference points. Survey vessels combine GPS positions with sonar depth soundings
to make the nautical charts that alert mariners to changing water depths and underwater
hazards. Bridge builders and offshore oil rigs also depend on GPS for accurate
Highly Accurate And Fast Process – The GPS technology supports the surveying process
by providing data with highest accuracy. This equipment is also faster when compared with
conventional surveying equipment. Because the data collection process is faster, the time for
getting final results and making decisions is shorter.
Time, Cost And Labor Saving Technique – The traditional and conventional surveying can
be very costly and time consuming process. In the past, surveyors had to make several visits
to one site in order to gather accurate data. With the GPS surveying this is no longer
necessary. This type of surveying reduces equipment and labor that was once required for
completion of a surveying task. A single surveyor can now complete all the tasks in one day,
what in the past took a whole team to do.
Not Affected By Weather Conditions – Another big benefit is that the GPS surveying is
not affected by weather conditions like snow, rain, high or low temperatures. Unlike the
traditional surveying techniques, the GPS surveying is not affected by constraints like the
line of site visibility between the survey locations.
The ongoing modernization and development of the GPS technology will make the GPS
surveying even more useful and widely adopted technique in the near future.
GPS surveying relies on receiving information from satellite signals. If anything
interferes with signal reception, the GPS results can lose accuracy. Tree coverage,
building and other tall structures, whether natural or manmade, may block satellite
signals from reaching the GPS receiver. Electromagnetic interference and even radio
signal interference may also disrupt GPS accuracy. Satellite signal reception also
means that GPS cannot be used when surveying underground areas or any property
blocked from a direct view of the sky.
A satellite constellation is a group of satellites working together to provide signal
coverage for a designated area. Any change in the satellite constellation's positioning may
affect GPS accuracy. Because satellite positioning is usually best during the normal 9-to-
5 workday, taking GPS surveying data outside of these peak hours may compromise
results. Such time constraints can lead to scheduling difficulties.
Conventional surveying produces data that can be divided into horizontal and vertical
measures, while GPS surveying delivers a fully three-dimensional representation, making
it impossible to separate the horizontal from the vertical. In GPS surveying, changing the
horizontal measure changes the vertical and vice versa. Fixed-height antennas can help
alleviate the problem, eliminating the need for height adjustments at each new location.
However, errors in measuring antenna heights can plague many GPS surveys.
Measuring elevation, or a point's height above a gravity surface, is the most significant
flaw in GPS surveying. GPS is incapable of directly measuring differences in elevation.
Surveyors must instead use an accurate geoid model, which depicts the shape of the
world's oceans under gravity alone, and at least four established benchmarks to determine
a point's elevation. The added effort may prove frustrating and time-consuming for many.
In addition, errors in antenna height, baseline measurements and control point locations
can further undermine elevation accuracy.
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