TECHNIQUES IN BASIC SURVEYING

Patel Dixit K. (130400106054)

Patel Dhruv P.(130400106051)

Civil Engineering departmentSankalchand Patel college of Engineering,

Visnagar

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.

BaselineA B

The basic principle of triangulation

Measured angle Measured angle

Unknown position

The basic principle of resection

Measuredangle

Measuredangle

Known position

Known positionKnown position

Measuredangle

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

BaselineA B

The basic principle of trilateration

Measured sideMeasured 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:

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

60º 60º

The equilateral triangleAll sides equal in lengthAll angles equal (= 60º)

The isosceles triangleTwo sides equal in lengthTwo 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.

Similar triangles have corresponding angles equalbut are of differing size

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.

Any closed polygon can be subdivided into 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 control points 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.

Baseline

ILLUSTRATION OF THE USE OF OFFSETS

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

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

ah

h

Correcting for horizontal distance:the “hypotenusal allowance”

a

h

z

correction factor = xy - yz

= xy(1 - cosa)

y

x