A2.2NP1 Environmental Practical 1

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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:

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### A2.2NP1Environmental 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.

The basic principle of triangulation

Measured angle

Measured angle

B

A

Baseline

The basic principle of resection

Known position

Known position

Measuredangle

Unknown position

Measuredangle

Measuredangle

Known position

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

The basic principle of trilateration

Measured side

Measured side

B

A

Baseline

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:
Plane Surveying: Theory
• 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

The equilateral triangle

All sides equal in length

All angles equal (= 60º)

60º

60º

The isosceles triangle

Two sides equal in length

Two angles equal

a

a

Plane Surveying: Theory
• 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.

Conguent triangles are identical

• two angles and one side equal
• two sides and one angle equal
• all three sides equal
Plane Surveying: Theory
• 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.
Plane Surveying: Theory
• 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.

These should be chosen to make as many of the triangles asclose 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

Plane surveying: practical aspects

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 controlpoints by triangulation or trilateration from the base-line.

Plane surveying: practical aspects

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.

Plane surveying: practical aspects

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
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
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

correction factor = xy - yz

= xy(1 - cosa)

x

h

a

y

z

### 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

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 absolute (datum) level of point A is 100.522m

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

The reduced level of point B is 100.301m

A

B

2.312 m

2.533m

IP 1

Datum line: 100.522m

(from OS Benchmark)

Rise and fall booking

Backsight

Interm.

Foresight

Rise

Fall

R.L.

Distance

Remarks

-

Point A

2.312

100.522

2.533

Point B

0.221

100.301

1.2

Transfer of level

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

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

The absolute level of point C is 100.344m

B

C

1.674 m

1.631m

IP 2

Rise and fall booking (cont)

Backsight

Interm.

Foresight

Rise

Fall

R.L.

Distance

Remarks

-

Point A

2.312

100.522

Point B

1.674

2.533

- 0.221

100.301

1.631

+ 0.043

100.344

Point C

1.2

Continuing this process, suppose we end up with a set of results as follows:
• This will enable us to check our working
Rise and fall booking (cont)

Backsight

Interm.

Foresight

Rise

Fall

R.L.

Distance

Remarks

--

--

--

Point A

2.312

100.522

--

Point B

1.674

2.533

- 0.221

100.301

--

2.504

1.631

+ 0.043

100.344

Point C

--

0.956

+ 1.548

101.892

3.010

--

2.413

2.016

+ 0.994

102.886

--

2.718

-0.305

102.581

--

11.913

9.854

2.585

- 0.526

102.581

9.854

- 0.526

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

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

The absolute level of the intermediate point C is 98.987m

B

C

Intermediate sight

1.674m

2.988m

IP 2

Rise and fall booking (intermediate sight)

Backsight

Interm.

Foresight

Rise

Fall

R.L.

Distance

Remarks

-

Point A

2.312

100.522

Point B

1.674

2.533

- 0.221

100.301

2.988

-1.314

98.987

channel

1.631

+ 0.043

100.344

Point C

Next FS

1.2

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:

Multiply vertical

distance by 100

to obtain

horizontal distance

lines

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

Tacheometric distance

Change of height

Measured angle

a

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.