1. Introduction

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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).

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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.

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1. Introduction

• Geodetic surveys allow for curvature of Earth
• 12500 scale maps actually vary in scale
• On extreme east and west coasts, scale is
  approximately 12501
• Reference is taken along 2 oW where scale is
  12499
• Along Greenwich Meridian and 4oW scale is 12500

• 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
• .

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2. Basic Surveying Methods

• Point location - radial line and distance method.
  .

Difference in the Easting (?E) is given by-
Northing difference (?N)is given by-

True co-ordinates of the second point-
Easting Northing
This method can ONLY be used if there is an
INBUILT reference direction in the instrument -
e.g. magnetic north
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2. Basic Surveying Methods

• Point location - radial line and distance method.
  .
• No inbuilt reference

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

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2. Basic Surveying Methods

• Point location - Resection

Internal Triangle of Errors
External Triangle of Errors
Coordinates of A and B are known Point C found
from bearings at A and B or bearings from C to A
and B
Methods to distribute errors are needed
P
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2. Basic Surveying Methods

• Point location - Traverse Methods

Open Traverse Errors accumulate
Closed Traverse Errors can be distributed
Closed Loop Traverse Errors can be distributed
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2. Basic Surveying Methods

• Point location - Offset Methods

Useful for mapping features Not suitable where
accuracy is required
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2. Basic Surveying Methods

• Height Measurement

• Level Ground
• Base Accessible

• Level Ground
• Base Not Accessible

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2. Basic Surveying Methods

• Height Measurement

• Sloping Ground
• Base Accessible
• Observations to same height above ground

• Sloping Ground
• Base Accessible
• Observations to different height above ground

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2. Basic Surveying Methods

• Sloping Ground
• Base Accessible
• Base and Top above and below observer

• Height Measurement

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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.

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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.

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3.2 Basic Requirements

• 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?

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3.2 Basic Requirements

• Mapping vegetation boundaries
• accuracy 11000 (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.

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3.2 Basic Requirements

• 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!!

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3.2 Basic Requirements

• 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.

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3.2 Basic Requirements

• 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?
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3.2 Basic Requirements

• 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????

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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
• Bookers Name
• Observers Name

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3.4 Permissible Errors
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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

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3.5 Treatment of Errors

• Snedecors Rule

Where R is range of readings
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3.5 Treatment of Errors

• 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

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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
• 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

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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

• measured radial lines at A

• moved to B

• measured B to A

• measured radial lines at B

• moved to C
• etc

Despite careful planning points B, C, D and E
could not be located!
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3.7 An example of Bad Planning
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