TECHNIQUES IN BASIC SURVEYING -...

A2.2NP1

Environmental Practical 1

TOPIC 1

TECHNIQUES IN

BASIC SURVEYING

Basic ideas

• Surveying - the creation of a scale representation of

the ground surface - is a basic activity in many areas

of environmental management.

• A survey will be one of of two types:

– Primary survey - to establish the position of objects in three

dimensions when no previous information exists

– Secondary survey - to add extra information to existing

data or to measure changes over an interval of time

Basic ideas

• The task of three dimensional position fixing

is normally broken into two parts:

• Determining plan position

• Determining elevation

Basic ideas

• Each of these determinations may be either:

• absolute - made in terms of a fixed co-ordinate

system

• relative - made in terms of local co-ordinates which may later be converted to absolute co-

ordinates if required.

• The majority of surveys carried out for

environmental management are thus

secondary relative surveys

Plan Position Fixing

• The plan position of a station can be

established in a number of ways:

• By reference to the apparent positions

of astronomical objects when viewed

from that station

• This method gives the absolute location of the

station in terms of latitude and longitude,

which can be converted to local systems such

as the National Grid.

• By the measurement of the angles between lines

of sight to the unknown station from other known

positions

• By the intersection of lines of sight from the

unknown station to other objects whose

positions are already known

• These two methods both rely on the simple Euclidean

geometry of the plane. (Hence the term plane surveying).

The first procedure is termed triangulation and the second resection.

Baseline A B

The basic principle of triangulation

Measured angle Measured angle

Unknown position

The basic principle of resection

Measured angle

Measured angle

Known position

Known position Known position

Measured angle

• By measurement of distances between

the unknown station and other objects

of known positions

• This last method includes a number of

particular cases:

• measurements of offset distances from

a base line.

• trilateration - the distance equivalent of

triangulation.

• tacheometry - an optical method of

distance measurement along a known

bearing

Baseline A B

The basic principle of trilateration

Measured side Measured side

Plane Surveying: Theory

• Plane surveying relies on the basic

concepts of Euclidean geometry, and in

particular the properties of triangles.

• The most important (for our purposes)

of these are:


• The internal angles of a triangle sum to 180°

• The sides of an equilateral triangle are equal

and the internal angles are all 60°

• The base angles and opposing sides of an

isosceles triangle are equal

60º

60º 60º

The equilateral triangle

All sides equal in length

All angles equal (= 60º)

The isosceles triangle

Two sides equal in length

Two angles equal

a a


• If the respective angles in two triangles are

equal then the triangles are similar and their

sides are all in the same proportion

• If two triangles have two angles and one side

equal (or vice versa) then they are congruent

and all their other respective angles and

sides are equal.

• Two triangles are also congruent if all their

sides are equal.

Similar triangles have corresponding angles equal

but are of differing size

Conguent triangles are identical

• two angles and one side equal

• two sides and one angle equal

• all three sides equal


• Congruent triangles are unique - you cannot

draw two different triangles from the same set

of measurements

• This means that a complete set of survey

data must define the positions of objects

uniquely.


• Any closed polygon can be subdivided into a

series of contiguous triangles

• These properties are repeatedly used in the

procedure of triangulation in which stations

are surveyed in a pattern of contiguous

triangles.

Any closed polygon can be subdivided into contiguous triangles

These should be chosen to make as many of the triangles as close to equilateral as possible

Plane surveying: practical aspects

• In practice, most plane surveys are carried out

in a straightforward way following an

established sequence:

1. A reconnaisance survey will establish the

dimensions of the area, relative levels,

significant features, accessibility, obstacles etc


2. Establish an accurate baseline by measurement

from existing survey points, natural features,

buildings etc. If none are available then the baseline

must be fixed by absolute methods.

3. Establish as required any further control points by

triangulation or trilateration from the base-line.


4. Incorporate detail by tacheometry, traversing, tape &

offset or whatever other method is appropriate.

5. The intermediate stations should where appropriate

be cross-checked with the control points by resection

and all traverses should be closed at a control point.

6. Inaccessible detail should be incorporated by

triangulation or plane tabling from the ends of the

baseline.

Baseline

ILLUSTRATION OF THE USE OF OFFSETS


7. If a topographic survey is being undertaken, levelling

traverses should be carried out around the survey

stations and the baseline tied to the local benchmark

by a closed traverse.

8. The use of a theodolite or total station will enable both the position and the elevation of stations to be

found simultaneously by combined tacheometry and

triangulation or by trilateration

THE “CHAIN” SURVEY

How to establish relative plan positions

Chain survey

• Simplest of all survey techniques

• Relies on linear measurements; slopes

>3o require some adjustment to technique

• Usually requires a clear line of sight

• The triangles used should be equilateral

or approximately so

Terminology

• Trilateration is the measurement of

sides of a triangle

• whereas triangulation refers to the

measurement of the angles of the

triangle

Basic equipment

• Ranging poles

• Survey pegs and ‘arrows’

• Chain & tape measure or other distance

measuring instrument

• Plumb line

• Compass

Chain survey components

• Base line: the longest line

• Chain /survey lines

• Survey stations

• Offset lines

Order of events

• “Range out” survey stations with ranging rods

• Establish base line and measure accurately

• Measure remaining distances between other

survey stations

• Measure offset lines whilst measuring

between survey stations

Sloping ground

• If the ground slopes by more than about 3°,

this must be allowed for in the survey.

• The measured distances are thus slant

distances and must be corrected to true

horizontal distances.

• This requires that the vertical angle between

the stations is known

Ground distance determined

a

h

X

Sloping ground

• For an approximate survey, it may be

sufficient to step up or downhill using a

series of horizontal and vertical lines

• If the drop is measured at the same

time, some estimate of the slope profile

can be obtained

Chain surveying (“stepping”)

w

x

y

z

c

b

a

Sloping ground

• If stepping is not appropriate, more

sophisticated methods must be used to

measure the slant distance and the

vertical angle simultaneously

• Requires optical sighting equipment:

usually either a clinometer, Abney level

or theodolite

Basic levelling in chain surveys

a h

h

Correcting for horizontal distance:

the “hypotenusal allowance”

a

h

z

correction factor = xy - yz

= xy(1 - cosa)

y

x

LEVELLING

How to destermine relative

elevations

Levelling:

accounting for slopes

Unlike chain surveys, levelling surveys

account directly for slope and

incorporate this data into the whole

measurement exercise

AIMS:

• to determine height differences between

two points

• to determine elevations for sections

• The elevation of a station can be

established by:

• inclined line of sight from chain survey stations

• levelling from another point of known height

• by inclined tacheometry

• Levelling is the more accurate method but is

also the slower. Modern instruments are

capable of cm accuracy under normal

conditions over distances of 100’s metres.

• The keys to successful levelling lie in the

setting up of the instrument, in the closure of

the traverses and in the careful recording

(booking) of the results.

• Inclined tacheometry relies on the combined

measurement, by theodolite, of the slant

distance to the new station and the angle

relative to the horizontal.

• The elevation change and horizontal distance

can then be found by simple trigonometry.

• The accuracy of the method, using normal

instruments, is around 10’s cms in 100’s

metres.

Direct levelling

• Most typical form used

Relies upon:

• a horizontal line of sight, also termed

“the line of collimation”

• a fixed datum level

Measurements to be taken

• Backsight

• Foresight

• Intermediate sights

Booking your results

The “rise and fall” method

• This method records the relative change

in level between successive stations

• The changes are converted to the

reduced level of each station

• The reduced level is relative to the local

datum

Booking the results

• The method relies on recording your

results in a survey book in a standard

format

• This allows you to check your work and

to identify any errors systematically

Reduced levels

The change of level is 2.312m - 2.533m = -0.221m

2.533m

Datum line: 100.522m

(from OS Benchmark)

2.312 m

The reduced level of point B is 100.301m

B A

The absolute (datum) level of point A is 100.522m

IP 1

Backsight Interm. Foresight Rise Fall R.L. Distance Remarks

2.312 100.522

0.221 100.301

1.2

Rise and fall booking

Point A

Point B

- 2.533

Transfer of level

The new change of level is 1.674m - 1.631m = + 0.043m

1.631m

1.674 m

The absolute level of point C is 100.344m

C B

At the next stage, B becomes the backsight and C is the new foresight

IP 2


2.312 100.522

- 0.221 100.301

1.2

Rise and fall booking (cont)

Point A

Point B

- 2.533 1.674 1.631 + 0.043 100.344 Point C

• Continuing this process, suppose we

end up with a set of results as follows:

• This will enable us to check our working


2.312 100.522

- 0.221 100.301

Rise and fall booking (cont)

Point A

Point B

--

2.533 1.674 1.631 + 0.043 100.344 Point C 2.504

3.010 2.413

0.956

2.016

2.718

-- --

--

--

+ 1.548

+ 0.994

-0.305

101.892

102.886 -- --

--

--

102.581

11.913

9.854

9.854 2.585

- 0.526

- 0.526 102.581

- 100.522

2.059 2.059 2.059 CHECKS OK

Using an intermediate sight

• Sometimes we wish to include a specific

feature but it is not convenient to set up

a new instrument position for this

• The solution is to take a sighting onto

the staff when it is placed on this feature

- this is called an intermediate sight

Intermediate sight

The new change of level is 1.674m - 2.988m = -1.314m

2.988m

The absolute level of the intermediate point C is 98.987m

C B

The intermediate sight is taken at the base of the channel between B and C

IP 2

Intermediate sight 1.674m


2.312 100.522

- 0.221 100.301

1.2

Rise and fall booking

(intermediate sight)

Point A

Point B

- 2.533 1.674

1.631 + 0.043 100.344 Point C

2.988 channel -1.314 98.987

Next FS

Optical distance measurement

• It is often convenient to use the levelling instrument itself to calculate the distance between the instrument and staff positions

• This is done using the stadia lines that are visible in the viewfinder

• These are arranged such that the distance to the staff is 100x the stadia interval that is read on the staff between the two lines

• This procedure is known as tacheometry

Tacheometry

The viewfinder:

Stadia

lines

Multiply vertical

distance by 100

to obtain

horizontal distance

Inclined tacheometry

• If the ‘level’ can be swung in a vertical

arc, the distance up an inclined sight

line can be obtained.

• If the vertical angle is also measured,

the slant distance can be converted to

give both the change in height and the

true horizontal distance.

Inclined tacheometry

a

Change of height

Tacheometric distance

Measured angle

True horizontal distance

The theodolite

• If such an instrument can also be swung in a

horizontal arc, and the angle of rotation can

be measured, we are able to determine the

angles of the sight lines between stations.

• This allows both trilateration and triangulation

with the same instrument.

• Such a versatile instrument exist and is called

a theodolite.

Summary

• Chain surveys are suited to planimetric

surveys on low slopes. They rely upon

trilateration.

• Levelling is used where terrain is more

uneven. Levelling surveys often use

tacheometry to fix station positions.

• A theodolite survey permits levelling,

tacheometry or triangulation as required.

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