Routing of Triangulation Survey
Consists of the following operations
o Reconnaissance/design
o Erection of signals/towers
o Measurement of baseline
o Measurement of horizontal angle
o Computations
Reconnaissance/design
The economy and accuracy of the whole triangulation system
depends upon an efficient reconnaissance
Reconnaissance
a) Examination of the area to be surveyed ( recall
general steps in control surveys)
b) Selection of Triangulation stations:
Should be intervisible
Placed upon the most elevated ground
Should form well-snapped triangle
Easily accessible
Line of sight do not pass over town etc
Serve as the control for possible future extension
Reconnaissance
c) Station marks
Should be permanently marked with copper or
bronze tablets
Name of station, year, should be stamped on the
tablet
Double beacon
Provide reference mark
Description card
Secure on a rock or concrete monument
Erection of signal and tower
Tower - is a structure erected over a station for
the support of the instrument and observing
part
Provided when the station or the signal or both are to be
elevated
Signal – is a device erected to define the exact
position of an observed station
Erection of signal and tower
Three types of signal exists:
1) Non-luminous/opaque signal, e.g. target
Used for direct sights less than 30k
2) Luminous/sun signal
Sun rays are reflected to the observing theodolite either
directly as from a beacon or indirectly from a signal target
3) Night signals
Oil lamp/electric lamp with reflectors
Base line measurement
Great accuracy of measurement is required
Length of baseline depends upon the grade of
triangulation net
Selection of site for baseline should:
Be fairly level
Be free from obstruction
The two points be intervisible
Suit extension to required triangulation network
Horizontal angle measurement
High precision instruments are to be used
a) Repeating theodolite e.g. T2, DKM2, etc
The optical micrometer reading of 1sec
Used for secondary and third order works
b) Direction theodolite e.g. T3, DKM3, etc
Micrometer reading less than 1 sec
Used for primary works
Horizontal angle measurement
Method of observation
a) Direction method
Several angles at station are measured in terms of the
direction of their side from that of an initial station
More than two targets
b) Repetition method
Each angle measured independently
TRIANGULATION COMPUTATIONS
OBJECTIVE
To determine the best possible planimetric position
(x, y) coordinates
Procedures of computations
Correct all baseline measurement for systematic
errors and reduce distances to mean sea-level
Reduce observed direction or angles to average
values and check triangle closure
Perform necessary reduction to centre when accentric
stations have been used.
Compute spherical access
Procedures of computations
Compute preliminary positions
Adjust triangulation network by a method of Least
square
Perform final position computation when necessary to
determine final coordinate
CONTROL SURVEY APPLICATION
a) Horizontal and vertical control densification
horizontal control densification
- conventional and GPS surveying
vertical control densification
- conventional leveling: for small project site
or construction site.
- differential GPS: for large mapping project
control survey application contn…
b) Structural deformation studies
Monitoring the motion of points on a structure
relative to stable monuments
Done using EDM or with GPS
control survey application contn….
c) Photogrammetry
Support photogrammetric mapping application
How?
Provide rigid horizontal and vertical alignment of the
photographs (orient the photos on the ground)
control survey application contn…
d) Cadastral surveying
Generally, accuracy standard for cadastral surveys
conform with third order point closure standard
control survey application contn…
e) Construction survey
In-house and contracted construction survey, must
meet third order – class II accuracy.
Stake out work for earthwork clearing and grading
may need to meet fourth order accuracy
Stake out work such as tunnel or bridge pier
alignment may require second order or higher
accuracy criteria.
control survey application contn…
f) GIS integration
Geographical position of natural and man-made
features should be accurate and stored
Enables the user to exchange information between
database
Conventional survey and GPS surveying used as a
foundation to establish the geographical coordinate.
