1. introduction • surveying method chosen depends...

1. Introduction

• Surveying Method chosen depends on:

• by the purpose of the survey e.g. map making,

location of specific points, definition of land

ownership etc.,

• by the nature of the survey itself e.g. hydrographic,

terrestrial, astronomic,

• according to the scale or accuracy of the survey,

• the type of instrument or instruments used

– e.g.

» prismatic compass,

» level

» theodolite,

» photograph (terrestrial or aerial).

1. Introduction

• Other factors:

• Curvature of earth

over 5 km, vertical angle difference will approach 2.5 minutes which

can readily be detected even with most basic theodolite.

• Errors

collimation errors in instruments need to be calibrated otherwise “rivers

could flow uphill”

Surveying involves

• transfer of levels between two points

• measurement of angles and lengths.

• requires solution of triangular shapes using basic trigonometry

(or by graphical means).

• If distances are large: planar geometry no longer applies.

• Geodetic surveys allow for curvature of Earth – 1:2500 scale maps actually vary in scale

– On extreme east and west coasts, scale is approximately 1:2501

– Reference is taken along 2 oW where scale is 1:2499

– Along Greenwich Meridian and 4oW scale is 1:2500

1. Introduction

• Surveying Instruments can be very accurate – Instruments in ENV are capable of accuracies of 1 part in 20000 with ease if

used correctly.

• Maps in UK are based on cartesian co-ordinates – North is represented by a bearing of 000.

– East a bearing of 090,

– South-west a bearing of 225 etc.

• Referencing: True North: Grid North: Magnetic North – .

• Point location - radial line and distance method. .

2. Basic Surveying Methods

Difference in the Easting (∆E) is given by:-

θθθθ==== sin.E l∆

Northing difference (∆N)is given by:-

True co-ordinates of the second point:-

Easting =

Northing =

θθθθ==== cos.N l∆

θθθθ++++ sin.Eo l

θθθθ++++ cos.No l

where is the length of the line,

and θ is the bearing

where Eo is Easting

and No is Northing

of the reference station.

l

N

O

lθ

This method can ONLY be used if there is an INBUILT reference

direction in the instrument - e.g. magnetic north

• Point location - radial line and distance method. .

• No inbuilt reference


N

O

lθ

R

Two horizontal angles are

ALWAYS needed

i.e. a reading to R (a reference

object) as well as object of interest

Applies to most instruments:

• Total Stations

• Theodolites

• Levels etc

• Point location - Resection


A

B

C N

Coordinates of A and B are known

Point C found from bearings at A

and B

or bearings from C to A and B

A

B

C

B

A

B A

C

C

P

D

External

Triangle of

Errors

Internal Triangle

of Errors

Methods to distribute errors are needed


• Point location - Traverse Methods

b a

c A1

b a

c A1

A2

b a

c A1

e

d

Open Traverse

Errors accumulate

Closed Traverse

Errors can be distributed

Closed Loop Traverse

Errors can be distributed


• Point location - Offset Methods

A B

2

1

D1

D2

d1

d2

Useful for mapping features

Not suitable where accuracy is required


θθθθ++++==== tandhH Ho

)(sin

sinsindhH

12

12Ho

θθθθ−−−−θθθθ

θθθθθθθθ++++====

• Level Ground

• Base Accessible

• Level Ground

• Base Not Accessible

• Height Measurement


θθθθ==== sindH S

1So hsindhH −−−−θθθθ++++====

• Height Measurement

θ

H

θ

H

• Sloping Ground

• Base Accessible

• Observations to same height

above ground

• Sloping Ground

• Base Accessible

• Observations to different height

above ground


)tantan(cosdH 122S2 θθθθ++++θθθθθθθθ====

2So1 sindhH θθθθ−−−−====

• Height Measurement • Sloping Ground

• Base Accessible

• Base and Top above and below

observer

H1

H2

3. Planning a Survey

• Careful Planning is needed

– A single missed reading will make whole survey of no

value

– Need to provide checks

– Abstract raw data in field

– Repeat readings if necessary before leaving site.

– Remember an extra set of readings may take 15

minutes - but to remobilise and set up again may take

many hours

– schedule breaks effectively.

3.2 Basic Requirements

• A clear statement of purpose of survey is needed

• e.g.

• mapping vegetation boundaries; estimating river bank plan

shape or erosion rates;

• determining flow characteristics in rivers;

• establishing fixed reference stations for future use;

• locating the point at which a particular set of measurements

have been taken;

• measurement of the profile of a slope;

• assessment of regions liable to flooding.

• scale of the map required (if relevant).

. The purpose of the survey will dictate the scale and

accuracy required and ultimately the methods to be used.


• Secondary Planning Requirements include:-

• what equipment is actually available

• what time is available

• what man-power is available

• what access and transport are available.

• over what distance will the surveying party be spread during

the surveying?

• Will it be necessary to return to the same site at a later date

to take repeat measurements, and if so when (within a few

days, or several weeks or months later?).

• How will contact between members of the surveying team

be maintained at distance?


• Mapping vegetation boundaries:

– accuracy ~ 1:1000 (1 m is represented by 1 mm),

– Suitable surveying methods:

• compass and tape traverses,

• chain and offset mapping,

• point resection using a prismatic compass.

• height variations, Abney levels will often be adequate.

– Alternative methods, if the equipment is available,

• use of a surveyor's level and tachymetery,

• use of electro-magnetic distance measurement.


• Water Slope Measurement

– Difference in water surface elevation in a river

is small,

– Measurement requires accurate measurement of

height differences over distances which are

usually between 10m and 500m apart.

– A good surveying level for which the

collimation error is known is required.

– Otherwise river may appear to flow uphill!!


• Fixed Control Points

– Measurements will be needed to the nearest millimetre

(centimetre) even if associated mapping detail is not

required at this level of accuracy.

– Sometimes, such as in the vegetation survey, simple

methods can be used including prismatic compasses to

establish stations,

– Control stations will be located more accurately using

a theodolite and associated equipment.


• Surveying River Banks

– Methods

• Radial Line Techniques using tachymetry for general plan

shape of meanders (general profile ~5m).

More accurate methods involving the establishment of short

permanent base lines on the bank parallel to the long stream

direction of the river are needed for erosion studies.

accurate profiles of bank are determined using metre rule

offsets from this reference line to the edge of the river bank.

Decisions needed

what constitutes the edge of the channel?

~10 - 20 m


• Size of Survey Party:-

• what equipment is to be used for accuracy

• access for vehicles

• e.g.

– a theodolite requires a tripod and targets

– may also require targets mounted on tripods

– could require a minimum of three tripods and ancillary

equipment.

• Often makes sense to establish control stations separate

from detailed purpose of survey

• Communication

– Radios

– Flags?

• Markers: Permanent: Temporary????

3.3 Booking of Data

• in waterproof notebooks

– should be logical

– should always be done in the field

– if necessary it can be transcribed BUT the ORIGINAL

BOOKING MUST ALWAYS be accessible.

– Cross checking should be booked in field

– Critical Information which should always be present

• Purpose of Survey: (River Bank Mapping at Maes Mawr)

• Date: Time: Weather:

• Specific Location

• Sketch of Area

• Booker’s Name:

• Observer’s Name

3.4 Permissible Errors Surveying Method Maximum Error

Prismatic Compass * 0.5 o

Tripod Mounted Compass * 0.05o

Angular Measurement with a Level 0.1 - 1.0o

Angular Measurement using a theodolite

(depends on instrument). 1 - 20 seconds

Level transfer using a Surveyor's Level 20 mm per km

Distance measurement using tape 1 part in 400+

Distance measurement using catenary taping 1 part in 1500+

Distance measurement using a subtense bar -

depends on configuration 1 part in 2000 - 10000

Electro magnetic distance measurement * 5 mm irrespective of distance

Closing error in a compass and tape traverse 1 part in 400

Closing error in a theodolite and EDM traverse 1 part in 10000

Closing error on a set of angles at one station

(20 second instrument) * 20 seconds

Closing error in a triangle (20 second

instrument) * 30 seconds

3.5 Treatment of Errors

• Systematic Errors

– collimation errors in instrucments

– magnetic errors affecting all readings at a particular location

– Systematic errors (unless large) can be compensated - will

always be present and calibration is important.

• Random Errors

– observer variations in reading a scale

148.1 148.2 148.0 148.1 148.1

148.1 148.2 147.9 148.1 148.2

– Mean is 148.1 - standard deviation is 0.09


• Snedecor’s Rule

Number of

Observations

Approximation of

Standard Deviation

5 R / 2

10 R / 3

25 R / 4

50 R / 4.5

100 R / 5

Where R is range of readings


• Random Errors

148.1 148.2 148.0 148.1 148.1

148.1 148.2 147.9 149.3 148.2

– Mean is 148.22 - should this value be used?

– Exclude 149.3 and mean is 148.10

– standard deviation is 0.10

– value of 149.3 is 12 standard deviations from mean

and should be excluded.

• Gross Errors should always be discarded

3.6 Provision of Checks

• Going to repeat a survey is time consuming

• Always provide a check

– pace out a distance

– sight on three positions rather than basic two

– if measuring three stations - take readings at all three

stations - sum of angles should be 180o

• For Level transfers

– e.g. elevation profile of longstream of river

• trace river from start to finish

• transfer level back along back - differences should be same

• if errors are unacceptable (e.g. 20 mm per 1 km) then repeat

until consistency is achieved

3.7 An example of Bad Planning

• ENV students doing their third year project in 1992 in Derbyshire

• needed to map meanders

• decided to use radial line method and tachymetry using level

• measured A to RO

Lady

Bower

Reservoir

A

B

C

D

E

RO

• moved to B

• measured radial lines at A

• measured radial lines at B

• measured B to A

• moved to C

• etc

Despite careful planning points B, C, D and E

could not be located!

3.7 An example of Bad Planning

A

B

C

RO

What should have been done?

Before leaving Derbyshire, map

should have been plotted.

Before moving from A to B

measure A to B

When at B measure from B to A

1. introduction • surveying method chosen depends...

Documents