MODERN TRENDS IN CONTROL SURVEYING

A control surveying is the framework of survey stations whose coordinates

have been precisely determined and are often considered definitive. The

stations are the reference monuments, to which other survey work of a

lesser quality is related. By its nature, a control survey needs to be precise,

complete and reliable and it must be possible to show that these qualities

have been achieved. This is done by using equipment of proven precision,

with methods that satisfy the principles and data processing that not only

computes the correct values but gives numerical measures of their precision

and reliability.Since care needs to be taken over the provision of control,

then it must be planned to ensure that it achieves the numerically stated

objectives of precision and reliability. It must also be complete as it will be

needed for all related and dependent survey work. Other survey works that

may use the control will usually be less precise but of greater quantity.

Examples are setting out for earthworks on a construction site, detail surveys

of a greenfield site or of an as-built development and monitoring many

points on a structure suspected of undergoing deformation.

The practice of using a control framework as a basis for further survey

operations is often called ‘working from the whole to the part’. If it becomes

necessary to work outside the control framework then it must be extended to

cover the increased area of operations. Failure to do so will degrade the

accuracy of later survey work even if the quality of survey observations is

maintained.

For operations other than setting out, it is not strictly necessary to observe

the control before other survey work. The observations may be concurrent or

even consecutive. However, the control survey must be fully computed

before any other work is made to depend upon it.

Fundamental Control Surveying:This is the control survey to determine the coordinates for field work of bridging and its

related at both beach sides. This is the first and second order trilateration method using

electromagnetic distance meter (EDM) and theodolite. Ten reading works in one set are

continued three times and the allowable deviation of reading distance in one set is

within 20mm in consideration of all meteorological data. The difference between actual

measured distance and checking survey is as follows:

Table 1. Distance Check (Spherical)

First Order Net

Second Order Net

Fundamental control survey was performed using the Range-master-III type, YHP

3808A Wild T3 and T2. After 1963, K+E Auto-ranger-JX and Sokkia Total Station SET

2EX were used instead of Range-master. The result of over-sea levelling using Wild N3

and T3 is as follows:

– The standard deviation of one observation value + 11.3mm

– The standard deviation of mean value + 1.9mm

At the both beaches the first order levelling was performed and the error was –0.6mm

and

–0.7mm compared to the national levelling points.

Control Survey of Main Tower and Cable:

The control survey of the main tower has been done in the factory and checked in

fabricating process each 30 step and used three dimensional comparator machine. The

several points are checked as each scaffolding step whether the position accuracy is kept

within 1/10,000 and finally at the top of the tower in the filed, accuracy of coordinates

was controlled within 28.3mm. Also vertically was surveyed by EDM system. This

operation was performed during midnight, sometimes continued up to the early morning

and finally placed within 1/5,000 of the tower lenght at the top of tower. In fact the

verticality error was +5.26mm in south direction and 0.06mm in east direction and

1.08mm as mean value in all direction and gained high accuracy result

The control survey of cable was performed using laser oscillators when it hanged over

the bridge. The figure shows the field works, A is laser oscillators and B is receiver. At

the center of cable, the sag is adjusted so as to keep the predetermined value in

consideration of temperature affection

TRIANGULATION:

Technique used in surveying to determine distances, using the

properties of the triangle. To begin, surveyors measure a certain length exactly to

provide a base line. From each end of this line they then measure the angle to a distant

point, using a theodolite. They now have a triangle in which they know the length of one

side and the two adjacent angles. By simple trigonometry they can work out the lengths

of the other two sides.

It is a technique based on principal angular measurements. Before the middle of the 20th

century, it was the most common method for establishing geodetic control networks and

for sole calculation of conspicuous points, marks and other aids to navigation or

photogrammetric ground control points. Since

the 1960s Electronic Distance Measuring equipment (EDM) or Electro Optical Distance

Measurement (EODM) has superseded the above methods. More recently they have been

replaced by satellite methods, particularly since a permanent global coverage was

established in the 1990´s The earliest form of triangulation for hydrographic purposes

consisted of a series of observations as in Fig. 6.3, with a relatively small number of

measured sides (baselines) and a large number of angular measurements, which are

showed here with the observed directions. Such a diagram provides a great deal

of redundancy; each double-diagonal quadrilateral has three angular verifications created

by adding or subtracting values. Nevertheless the network scale is still determined by the

baselines.

In old unconnected geodetic controls, position and orientation were established from

astronomic observations of latitude, longitude and azimuth in a datum. Now days, if

marks are use from these kinds of networks, it is usually necessary to re-observe and

recalculate via GNSS in order to convert coordinates into a universal system like WGS

84 (see 2.2.3 at Chapter 2).In general it should be noted that distances from the baselines

could be measured within accuracies ranging from 1ppm to 3ppm, directions from ± 0.5"

to ± 2", and transition from a base to another (that is, the contrast between the base

transfer by triangle resolution and the other measured base) could normally

be checked within 20 ppm and 40 ppm. These limitations should be taken into account

when trying to adjust an old triangulation network to a present framework via GNSS

observations, with distances of 200 or 300 km there can be differences of

several meters (2 or 3). Besides tolerating differences of these orders, it is necessary to

have a sufficient number of well-distributed connections to common datums and of

datum conversion algorithms in order to absorb the distortions typical of the old networks

(see Chapter 2) Not withstanding the above statement, densification by GNSS of datums

with fixed co-ordinates computed from old triangulations should be avoided; such cases

often lead to distortions and inaccuracies in the final results. If unavoidable due to the

need to keep the co-ordinates of an old datum, it will be necessary to adopt very

particular computation strategies and the limitations of the values obtained must be stated

at an early stage.

In lower order work, it was normal to select a few higher order points at a time, as in Fig.

6.4 left; though in some cases for control densification networks selection of a larger

number of points with shorter side observations (Fig. 6.4, right) was carried out,

particularly whenever triangulation towers had been removed. These towers were used to

elevate the line of sight over trees, topographic features and other obstacles interfering

with the observations. Obviously their removal prevented long sights from being

conducted which led to this type of solution.

In some cases, in hydrographic surveying, the term triangulation has been used to

describe survey points with flare triangulation by intersection (see 2.2.4). These flares

were dropped with parachutes vertically over the point to be located and, whilst burning,

directions from control points would be simultaneously observed; this was conducted

towards many coastal points requiring to be surveyed, as many were not

visible. Balloons, luminous shots or mobile, elevated signals were also used for the same

purposes.

The term triangulation has also been used when measuring angles to control points with a

sextant, sometimes in combination with observations from such points. The exclusive use

of observations from points to be calculated is treated as a resection in 2.2.4.

Although these survey techniques are becoming obsolete due to intensive use of other

more responsive modern methods, they still are effective.

One of the typical problems of triangulation is the error propagation dependence on the

figure shapes, on which the error results (positions) rely not only on the measurement

error but also on the network geometry. This problem is dealt with for particular cases in

2.2.4 though it admits more complex generalisations. For example, a single chain of

equilateral triangles is more rigid than a chain with unequal angles. Also, a two-diagonal

square chain is more rigid than a chain with rectangles or trapeziums with similar

diagonals.

The fundamental principles upon which the surveying is

being carried out are

working from whole to part.

after deciding the position of any point, its reference must be kept from at least

two permanent objects or stations whose position have already been well

defined.

The purpose of working from whole to part is

to localise the errors and

to control the accumulation of errors.

This is being achieved by establishing a heirarchy of networks of control points. The

less precise networks are established within the higher precise network and thus restrict

the errors. To minimise the error limit, highest precise network (primary network) Figure

1.1 of control points are established using the most accurate / precise instruments for

collection of data and rigorous methods of analysis are employed to find network

parameters. This also involves most skilled manpower and costly resources which are

rare and cost intensive. Further elaboration has been done in Lesson 3 under "Overview

of Land Surveying".

Working from whole to part:

In order to localize errors and prevent their accumulation, a set of control

points is always established with great precision first for the whole area to be

surveyed.

Later on, details or filled in between these control points to a relatively

smaller precision.

This fundamental work principle is known as “Working from whole to part”.

The first step in extensive surveys such as town or topographical surveys, is to

establish a system of control points with high precision.For horizontal control the

points are established by triangulation(by making a triangles) or by precise

traversing(by making polygons).

To do this by triangulation, the area to be surveyed is divided into large triangles

which are surveyed with the greatest accuracy.They are further divided into small

triangles which are surveyed with less accuracy.The object of this system of

working is to prevent the accumulation of error and to control the localize minor

errors.On the other hand if we work from part to the whole, small errors are

magnified in the process of expansion and become uncontrollable at the

end.Thus in this method of surveying by chain angles, the surveyor fixes the

position ofthe part of a line Bb in the figure 1 below and from that he finds the

direction of the whole line BC.This method is opposed to the above principle and

should be used only in exceptional circumstances.

TO FIX THE POSITIONS OF NEW STATIONS AT LEAST BYTWO METHODS:The new stations are fixed from points already fixed by,

i. Linear Measurement

ii.Angular Measurement

iii.Both Linear and Angular Measurements

TRIGONOMETRIC LEVELING:

When you know the vertical angle and either thehorizontal or slope distance

between two points, you canapply the fundamentals of trigonometry to calculate

thedifference in elevation between the points. That is thebasic principle of

trigonometric leveling. This methodof indirect leveling is particularly

adaptable to rough,uneven terrain where direct leveling methods

areimpracticable or too time consuming. As in any survey,the equipment that you

will use in trigonometric levelingdepends on the precision required. For

mosttrigonometric-leveling surveys of ordinary precision,angles are measured

with a transit, or alidade, anddistances are measured either with a tape or by

stadia,which you will study in chapter 8. On reconnaissancesurveys the vertical

angles may be measured with aclinometer, and distances maybe obtained by

pacing.The method used in trigonometric leveling …

ESTABLISHMENT OF THE TRIGONOMETRIC LEVEL NETWORKS:

Field works on three different locations have been done, to provide heights of

stabilized benchmarks on the central Adriatic islands. Zenith angles were measured

by electronic theodolites, and distances between points were obtained by total

station and GNSS measurements. Network points were stabilised on shortest

distances between island and mainland. Networks consist of two points on both

sides and an additional subsidiary benchmark. All distances were measured in

same time period with identical equipment.

Distances and angles were measured in both directions. Horizontal angles were

measured in four repetitions, two in each of the direct and reverse modes. For all

repetitions each point’s zenith angle was measured multiple times from both sides.

It was realised by measuring zenith angles 10 or more times, manipulating only

vertical tangent circle screw (horizontal circle was unchanged). Same procedure

was applied on all network points.

Integration in unique vertical system was realised by measuring closest vertical

control benchmark near stabilised points on the mainland and calculate their height

in the official vertical datum. Measuring height differences between points on a

same side by geometric levelling provided better precision and quality adjustment

results. The point location had to satisfy several parameters, like free horizon for

GNSS measurements, clear visibility between all network points for trigonometric

levelling etc.

Trigonometric levelling was realised by two instruments, total stations LEICA

TC2003 and LEICA TCA2003. Distances that could not be measured by total

station (large distance) were obtained using four GNSS instruments (Trimble R7).

Furthermore, GNSS measurements were used for evaluation of distances measured

by total stations.

GNSS/levelling was also applied and results were compared. For the purposes of

processing and adjustment of measurement and definition of the height differences

between network points the specially designed software for the adjustment of

measurements of this type were used in which is possible to arbitrarily define the

weights of individual measurements, and so this software allows specific analysis

of the impact of the weights on the results.

POSITIONING OF TRIGONOMETRIC LEVEL NETWORK

The precise positioning is essential not only for calculations of different

corrections but also to provide quality points for national survey database. Trigonometric

leveling networks were not necessary relied on the state control network because only

distances between points were measured. Therefore, precise positions and quality heights

are very important if they will be used in future geodetic works. The network point

adjustment is realised by two fixed points, CROPOS GNSS network referent stations. To

achieve relative position accuracy of +1cm+2ppm, and +1cm+1ppm for vertical accuracy

static GNSS relative method was applied (1 hour of static measurements).

Trigonometric levelling

Trigonometric levelling measurements were carried out in two or sometimes three

stages. The adjustment of multiple measured vectors had been made using Microsoft

Excel services to get final height differences before final adjustment.

The table 3 shows trigonometric levelling results with comparison of the heights,

standard deviations of the unknown points and referent standard deviations. These results

represent the quality and reliability of points on islands. The comparison of standard

deviations for adjusted unknown values from different stages and final adjustment

standard deviation is made. It is noticeable that additional stages of trigonometric

levelling reduced standard deviation of unknowns.