LIMITING LENGTH OF
OFFSET
Compass survey
Chain surveying can be done when area to be here staff will be used to measure
offsets surveyed is comparatively small and fairly flat.
> But when large area is to be surveyed then methods of
chain surveying alone are not sufficient and convenient. In such cases there is
need to use some instrument to measure directions or angles of line to be
surveyed.
> Following
instruments can be used be used for the purpose stated above.
a)
Instruments for direct measurements of directions i)
Surveyors compass i) Prismatic compass
b)
Instrument for measurement of angles \
i) Sentant
i) Theodolite
Traverse survey:
> Traversing is the type of survey
in which number of connected survey lines forms the framework
and the directions ofthese survey
lines are measured with the help of angle measuring instrument
and lengths of survey lines are
measured by chain or tape.
> When the survey lines forms a
circuit or traverse which ends at the starting point then it is called
as “closed traverse”
> When the circuit ends somewhere
else it is called as “open traverse”.
Compass surveying:
> Compass surveying is the branch
of surveying in which directions of survey lines are measured
by a compass (an angle measuring
instrument) and lengths of survey lines are measured with
chain or tape.
> It
is generally used to run a traverse or in traversing.
Units of angle
measurement:
There are three popular systems
of angular measurement.
1)
Sexagesimal system
1 circumference = 3600 (degrees of arc) 1
degree = '
60 (minutes of arc)
1 minute = ''
60 (seconds of arc)
> Widely used in Us, Britain,
India and other parts of world.
> Most ofthe surveying instruments
are graduated according to this system
2)
Centesimal system
1 circumference = 4000g (grads) 1 grad = 100C
(centigrade)
1 centigrade = 100CC
(centicentigrads)
> Used
in Europe due to easy computation and interpolation.
3) Hour system : (As per
rotation of earth)
1 circumference = 24h
(hours oftime)
1 hour = 60m (minutes
oftime)
1 minute = 60s
(seconds of time)
24 hr = 3600
1 hr = 150
60 min = 50 60 '
1 min = '
15
60 sec = 15 60 ''
1 sec = 15"
> Used in Astronomy and navigation.
Bearings and angles:
> Directions of survey lines are
established either with respect to each other or with respect to any
meridian.
> Thus, meridian is a fixed line
ofreference with respect to or about which directions or angles are
measured.
> When angles of survey lines are
measured with respect to meridian it is called as bearing of a
line.
> Meridian can be any one ofthe
following.
1)
True meridian
> It
is the line joining true north and true south. OR
> Direction oftrue meridian through
a point is established by astronomical observations.
2)
Magnetic meridian
> Line joining through magnetic
north and magnetic south. OR
> Direction of magnetic meridian
can be established by magnetic compass.
3)
Grid meridian
> For survey of state, the true meridian of
central place is taken as a reference meridian for whole state and is called as
grid meridian.
4)
Arbitrary meridian
> Meridian
taken in any convenient direction towards a permanent or prominent marks or
signals. > To determine relative directions of various
lines in a small traverse or small area.
Types of bearing:
1)
True bearing:
> True bearing ofa line is the
horizontal angle that it makes with true meridian.
> It is also known as
“azimuth”
> Since, true meridian is
fixed so true bearing is also a fixed quantity i.e. does not change with
time.
2)
Magnetic bearing or Bearing:
> Magnetic bearing is the
angle that the line makes with magnetic meridian.
> This is not a constant
quantity may vary with time > Measured with a magnetic compass. >
Generally adopted for small areas.
3)
Grid bearing:
> Is the horizontal angle that
the line makes with grid meridian
4)
Arbitrary bearing:
>
Arbitrary bearing ofa line is the horizontal angle that it makes with arbitrary
meridian. Designations of bearing:
1)
Whole circle bearing system (WCB) or Azimuthally system
> In WCB system, the bearing
ofa line is measured in clockwise direction with magnetic north (or
south)
> Values of WCB varies from 00
to 3600
> Prismatic compass graduations are based on WCB
system.
> WCB is measured clockwise with north in India
& UK.
Bearing of line OA = 01
Bearing of line OB = 02
Bearing of line OC = 03
Bearing of line OD = 04
2)
Quadrantal bearing system or reduced bearing system:
> Bearing of a line is
measured from north or south, whichever is nearer towards eastward or
westward direction.
> Both south and north are
used as reference meridians and the directions can be either clockwise
or anticlockwise depending upon
the position of line.
> The quadrant in which line lies need to be
mentioned.
> Bearing varies from 00 to 900,
called as RB.
> Observed by surveyors
compass.
RB of OA = N 0 E
RB of OB = N a E
RB of OC = N f3 W
RB of OD = N y W
Conversion of bearings
from one system to another
Conversion of WCB into
RB
|
Line
|
WCB between
|
Rule for R Bor QB
|
Quadrant
|
|
OA
|
00 and 900
|
N9E
|
I
|
|
OB
|
900 and 1800
|
S (180 – 9) E
|
II
|
|
OC
|
1800 and 2700
|
S (9 - 180 ) W
|
III
|
|
OD
|
2700 and 3600
|
N (360 - 9) W
|
IV
|
Conversion of WCB into
RB
|
Line
|
QB or RB
|
WCB
|
Quadrant
|
|
OA
|
NQE
|
9
|
I
|
|
OB
|
SaE
|
(180 – a)
|
II
|
|
OC
|
Sf3W
|
(180 +f3)
|
III
|
|
OD
|
NyW
|
(360 - y)
|
IV
|
Note: when a line points
towards north, East, West and South, it is written / called as Due Alorth, Due
East, Due West Due South
respectively
Thus, N00 => Due North
N 900 => Due East
=> $ 900 E
$ 00 => Due South.
N 900 W => Due West
Fore Bearing and Back
Bearing.
If the bearing of a line is
measured in the direction ofprogress of survey indicated by an arrow
then it is called as fore
bearing or forward bearing OR
If the bearing of a line A is
measured from A towards B, it is known as forward bearing or fore
bearing (FE).
If the bearing ofa line is
measured in the direction opposite to the progress of survey then it is
called as back bearing or
backward bearing. OR
If bearing of line AB is
measured from B towards A , it is called as backward or back bearing,
since it is measured in backward
direction.
FB of AB = 600 = θ
BB of AB = 180 + θ = 2400
Left to right direction of
survey
FB of AB = θ1
BB of AB = 180+ θ1
Right to left direction of survey
FB of BA = 180 + θ1
BB of BA = θ1
I)
Thus, it can be observed that FB and BB for a line differs
by 1800
BB = FB + 1800, when FB < 1800 BB = FB + 1800, when
FB > 1800
II)
FB and AB = NQE
BB of AB = SQW
If bearings are given as
quadrantal bearings then to convert fore bearing to back bearing it is
only necessary to change the
cardinal points by substituting N for S and E for W and vice versa,
numerical value of bearing
remains the same.
Important Note:
It is always better to draw
rough sketches either for conversion of are bearing system to another
or for working out BB from FB and
vice versa.
Calculation ofincluded
angle between two lines from bearing.
Case I: if bearing of a line
is measured from a common point.
Included angle between AB and AC
= Q2 – Q1
For Quadrantal bearing
α = Q2 – Q1
α = Q1 + Q2
included angle, α = 180 - ( Q1
+ Q2 )
α = 180 - (Q1 - Q2)
α = 180 - (Q2 + Q1)
Case II: Bearing of two lines
not measure from a common point
Q2 = Q1 + α
Included angle, α = ( Q1
+180 ) - Q2
Thus included angle = BB of
previous line – FB of next line.
Calculation of
bearings from angles:
> When traverse is run
included angles between successive lines have been measured, from which
bearings of all lines can be
calculated provided bearing of one line is known.
> In a closed traverse
clockwise angles will be obtained if we proceed round the traverse in
anticlockwise direction.
> Bearing of any line
(FB) Bearing of previous line +
Included angle 1800
> If the sum is more than 1800
then deduct 1800 and if the sum is less than 1800 then
add 1800.
The bearing of BC = Q2
= Q1 + (α - 1800)
The bearing of CD = Q3
= Q2 + (β -1800)
The bearing of DE = Q4
= Q3 + (γ -1800)
The bearing of EF = Q5
= Q4 + (ö -1800)
Magnetic Compass:
> Gives directly the magnetic bearings of line
> Principle of working: if a
long narrow strip of steel or iron is magnetized and is suitably pivoted
about its centre so that it can
oscillate freely about the vertical axis, it will tend to align itself in
the magnetic meridian at the
place of observation.
Earth’s Magnetic
Field and Dip:
> Lines offorce of earths
magnetic field runs generally from south to north.
> Near the equator lines of force are parallel to
earth surface
> The horizontal projections
of lines of force de the magnetic meridian.
> The angle made by lines of
force or magnetic needle with the earths surface is called angle of
dip or simply dip.
> These lines offorce or
northern end of magnetic needle deflects downward in the northern
hemisphere and downward towards
south in southern hemisphere.
> At a place near 700
North latitude and 960 west longitude dip will be 900
this place is called as
North Magnetic pole. Similar area
in magnetic southern is called as south hemisphere pole.
> At any other place,
magnetic needle will not point towards the north magnetic pole, but it will
take direction and dip in
accordance with the lines of force at that point
> At the equator dip of the
needle will be zero and it will remain horizontal since lines of force are
parallel to the earths surface at equator.
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