data capturing techniques - total station and gps

1. Lecture – 3
Data Capturing Techniques

2. Lecture Objective
• after completing the lecture the students are
expected to
– Understand different data capturing methods
– Identify advantages and disadvantages of different data
capturing techniques

3. Data Capturing Techniques
1. Traditional methods
2. Compass and tape
3. Total station
4. GPS
5. Aerial Photographs
6. Satellite image

4. 1. Traditional methods
• Traditional methods vary within countries.
• They typically consist of a relatively simple
method for measuring parcel boundaries.
• Common examples would be to use ‘kada’ and
the rope method which utilizes a rope of known
length (e.g., 50 m, 100 m) to measure boundaries
of parcels.
• Areas are determined using simple mathematical
methods (areas of rectangles and triangles) with
assumptions on parcel shape.

5. 2. Compass and tape
• Tape: 30 m, 50 m, 100m
• Error source: inaccuracy in the length of the
tape, temperature, pressure, slope, sag

6. 3. Total Station
• Total station instruments, combine three basic
components:
– an electronic distance measuring (EDM)
instrument
– an electronic angle measuring component, and
– a computer or microprocessor-into one integral
unit.
Leica 1200 total station

7. What Total Station Does?
• Can observe horizontal and vertical angles, as well as
slope distances from a single setup.
• From these data they can instantaneously compute
elevations and coordinates of points sighted and display
the results on a display.
• Can also store the data, either on board or in external
data collectors connected to their communication ports.
7

8. Function of Total Station
• Averaging of multiple angle and distance
measurements
• Correcting electronically observed distances for
prism constant, atmospheric pressure and temp.
• Calculating point elevation
• Computing coordinates from horizontal angle
and distance
• Leica TPS1200+ model has a super accuracy of 1
mm + 1.5 ppm for a range of 3 km.

9. Example
• If an EDM has a standard error of ± (2mm+2ppm ) , what would
be the expected error in measuring a 1000 meter line?
solution:-
• standared error= ± (2mm+2ppm )
= ±(2mm+2/1,000,000(1000m))
= ±(2mm+0.002m)
= ±4mm

10. Control point establishment-Traversing

11. Detail Measurement

12. GPS for Cadastre
• GPS, can be used depending on the project
requirements, location, and other factors.
• GPS system, however, seems to be the most
suitable method, especially in unobstructed
areas.
• Inaccessible locations or obstructed areas can be
surveyed with total station.

13. Advantages of GPS
• There are several advantages of using GPS for
cadastral surveying.
• The most important one is that inter-visibility
between the points is not required with GPS .
• The accuracy obtained with GPS is reliable over
the entire network.
• Also, with GPS, one reference station can support
an unlimited number of rover receivers.

14. 1. Point positioning
• The receiver gets the coordinates of the
satellites from the navigation message while
the Pseudorange are determined from C/A
code
GPS Positioning

15. Relative Positioning
• The key point of relative positioning is to keep
the coordinates of the reference station fixed.
• provide a post processing.
• GPS relative positioning provides a higher
accuracy (cm level)
• Two satellite contain more or less similar errors
or biases.
• The shorter the distance between the two
receivers, the more similar the errors are.

16. Static GPS
• This method is used to give high precision over long baselines
such as are used in geodetic control surveys.
• One receiver is set up over a station of known X, Y, Z coordinates,
preferably in the WGS 84 reference system, and the second
receiver occupies the station whose coordinates are required.
• Accuracies in the order of 5 mm ± 1 ppm of the baseline are
achievable as the majority of error in GPS, such as clock, orbital,
atmospheric error and SA, are eliminated or considerably
reduced .
• Main purpose of static method are:
– establishing high precision control networks.
– it is used in control point densification.
– measuring plate movement in crustal dynamics .
– Monitoring structural deformation.

17. Rapid Static GPS
• One receiver always remains on a control station
while the others are moved progressively from one
unknown point to the next during the entire
observation session.
• The observation time or the occupation time for
rapid static surveying is relatively small.
• Collects data for about 2 to 10min depending on
the distance to the base and the satellite
geometry.

18. Stop and go GPS
• The data is usually collected at 1-to-2 seconds
recording rate for a period of 30 seconds per each
stop.
• The survey starts by first determining the initial
integer ambiguity parameters, a process known as
receiver initialization.
• Once the initialization is performed successfully,
centimeter-level positioning accuracy can be obtained
instantaneously.
• A special case of Stop and Go GPS surveying is known
as kinematic GPS surveying.
• Both methods are the same in principle.

19. RTK GPS
• The previous methods that have been described all require post-
processing of the results.
• But, RTK provides the relative position to be determined
instantaneously as the roving receiver occupies a position.
• The essential difference is in the use of mobile data /radio
communication to transmit information from the reference point
to the rover.
• Indeed, it is this procedure that imposes limitation due to the
range over which the communication system can operate.
• The system requires two receivers with only one positioned over a
known point.
• A static period of initialization will be required before work can
start.
• If lock to the minimum number of satellites is lost then a further
period of initialization will be required.
• Therefore the surveyor should try to avoid working close to major
obstructions to line of sight to the satellites.

20. RTK GPS
• The base station transmits code and carrier phase data to the rover.
• On-board data processing resolves the ambiguities and solves for a
change in coordinate differences between roving and reference
receivers.
• The great advantage of this method for the engineering surveyor is
that GPS can be used for setting-out on site.
• The setting-out coordinates can be entered into the roving receiver,
and a graphical output indicates the direction and distance through
which the pole-antenna must be moved.
• The positions of the point to be set-out and the antenna are shown.
When the two coincide, the centre of the antenna is over the
setting-out position.

21. • Methods for cycle slip recovery in true
kinematic mode are
– use of redundant satellites (≥ 4 four satellites),
– use of dual frequency data, and
– use of code/carrier combination