CONTROL SURVEYING.
Control Surveying is
the determination of the precise position of a series
ofstations distributed over an area to serve as the
origin or reference to be used forchecking of
subsequent surveys to be used in engineering
projects like propertydelineation, topographic and
hydrographic mapping, and construction planning
anddesign. Control networks which cover the
whole country have become been conductedwith
better accuracy and less stringent technique, with
the use of artificial satellites.These stations are
linked to local networks which have been adopted
for specialsurveys connected with projects such as
dams, roads, railways and pipelines, large orsmall
construction sites, etc. The purpose of a control
system is to prevent theaccumulation of errors, by
connecting detail work to a consistent geometrical
systemof points, which are accurate enough for
the project. Great care is taken to ensure thatthis
control is sufficiently accurate.
There was a time when geodetic control points consisted of triangulation
networksmarked by observation pillars. In the Philippines, many triangulation stations
havebeen located on top of towers which had been placed on mountain summits to
answerthe problem of intervisibility. But because of the ease with which positions can
beestablished by satellite systems, which eliminated the intervisibility requirement,
therehas been less need for establishing so many points. Gradually, as the scope of
thesurvey becomes smaller, the use of non-satellite systems to provide control
becomesmore prominent.Control underground, in urban streets and inside buildings is
predominantlycarried out by terrestrial methods. Apart from their use with further
ground surveyoperations, control points are also required to augment photogrammetric
and remotesensing methods of mapping. Plan coordinates and heights of points
identifiable onimagery are needed by all but the most sophisticated systems.
HORIZONTAL CONTROL POINTS TECHNIQUES.
A. TRIANGULATION (TRIGONOMETRICAL SURVEY)
 In the past it was difficult to accurately measure very long distances, but it was
possible to accurately measure the angles between points many kilometres
apart, limited only by being able to see the distant beacon. This could be
anywhere from a few kilometres, to 50 kilometres or more.
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 Triangulation is a surveying method that measures the angles in a triangle formed
by three survey control points. Using trigonometry and the measured length of
just one side, the other distances in the triangle are calculated. The shape of the
triangles is important as there is a lot of inaccuracy in a long skinny triangle, but
one with base angles of about 45 degrees is ideal.
 Each of the calculated distances is then used as one side in another triangle to
calculate the distances to another point, which in turn can start another triangle.
This is done as often as necessary to form a chain of triangles connecting the
origin point to the Survey Control in the place needed. The angles and distances
are then used with the initial known position, and complex formulae, to calculate
the position (Latitude and Longitude) of all other points in the triangulation
network.
 Although the calculations used are similar to the trigonometry taught in high
school, because the distance between the survey points is generally long
(typically about 30 kilometres) the calculations also allow for the curvature of the
Earth.
 The measured distance in the first triangle is known as the ‘Baseline’ and is the
only distance measured; all the rest are calculated from it and the measured
angles. Prior to the 1950s, this initial baseline distance would have to be very
carefully measured with successive lengths of rods whose length were
accurately known. This meant that the distance would be relatively short
(maybe a kilometre or so) and it would be in a reasonably flat area, such as a
valley or plain. The triangles measured from it gradually increased in size, and up
onto the hilltops where distant points could be seen easily.
FIGURE 1: TRIANGULATION NETWORK
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 The angles in the triangles are measured using a theodolite, which is an
instrument with a telescope connected to two rotating circles (one horizontal
and one vertical) to measure the horizontal and vertical angles. A good quality
theodolite used for geodetic surveys would be graduated to 0.1 second of an
arc and an angle resulting from repeated measurements would typically have
an accuracy of about 1 second of arc, which is equivalent to about 5 cm over a
distance of 10 kilometres.
STEPS IN TRIANGULATION WORK:
a) Reconnaissance
b) Measurement of Base Lines
c) Erection of Signals and Towers
d) Measurement of Angles
e) Astronomic Checks
f) Office Computations
B. TRILATERATION
 In the 1950s, accurate methods of measuring long distances (typically 30 to 50
km) were developed. They used the known speed of light (299,792.458 km per
second) and the timed reflection of a microwave or light wave along the
measured line. Known as Electromagnetic Distance Measurement (EDM), the
two initial types of instrument were the ‘Tellurometer’, which used a microwave,
and ‘Geodimeter’, which used a light wave.
 The distances in a triangle could then be measured directly instead of
calculating them from the observed angles. If needed the angles could be
calculated. The process of calculating positions through the chain of triangles is
then the same as for triangulation.
 Sometimes both angles and distances were measured in some triangles to check
on the observations and improve the accuracy of the calculations.
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FIGURE 2: TRILATERATION NETWORK
ILLUSTRATIVE PROBLEMS1. In a triangulation, an eccentric station is occupied instead of the true station A.
Observations then are made to true station A and to station B. The observation
are as follows:
LINE
Ecc. A- True A
Ecc. A-B
AZIMUTH
DISTANCE
158°30’50”
4.50 m
216°43’20.5”
18642.00 m
Find the azimuth of line thru A to B.
Solutions:
Using Cosine Law:
(AB)2 = (4.50)2 +(18642)2 -2(4.50)(18642) cos 58°12’30.5”)
AB= 18639.63m
Using Sine Law:
sinØ/ 18642 = sin58°12’30.5”/ 18639.63
Ø= 58°13’12.82”
Azimuth= 338°30’50”- (180°-58°13’12.82”)
Azimuth= 216°44’02.82” (azimuth of AB)
2. The lengths of the sides of triangle ABC, as measured with an EDM instrument are
AB= 923.245m, BC= 517.328m, and CA= 896.126m. The azimuth from the south of
side CB is 340degrees 15mins 20secs and the coordinates of station B are Xb=
10150.022m and Yb= 9450.085m. Assuming that the lengths are free from
systematic errors, determine the following preliminary data which are to be used
in subsequent adjustment by trilateration.
a) Interior angles of each side of the triangle
b) Azimuth from the south of sides AB and CA
c) Coordinates for points A and C
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means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.
Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any
means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.
Property of and for the exclusive use of SLU. Reproduction, storing in a retrieval system, distributing, uploading or posting online, or transmitting in any form or by any
means, electronic, mechanical, photocopying, recording, or otherwise of any part of this document, without the prior written permission of SLU, is strictly prohibited.