Choice of Scale Field
Book
·
In this book all detailing of survey are recorded.
·
Dimensions: 20 cm x 21 cm.
·
For
ordinary field work double line field book is used whereas for very large scale
work single line field book is used.
2. Linear
measurements including chain survey
There are various methods for
linear measurements.
1. Direct Measurement
Distances are actually measured on the ground. Methods:
1)
Pacing
2)
With the help of passometer – watch like instrument -*
(registers no. of paces)
3)
With
the help of pedometer: registers number of paces and adjusted to a length of
pace of a person carrying it - gives distances directly.
4)
By odometer or speedometer-*(speed and distance):
odometer measures the no of revolutions of a wheel. This number is multiplied
by circumference to get distance. Speedometer works on these principles.
Odometer is fixed to the wheel.
5)
Chaining:
measuring distances with the help of chain or tape. This is the most accurate
method of measuring distances.
2. Measurement by
optical means
Observations are taken through telescope and
calculations are done for distances, such as tacheometry.
3. Electro – Magnetic
Methods
Instruments are used for indirect measurement
ofdistances which works on the principle of propagation, reflection, and
subsequent reception of either radio waves, light waves or infrared waves.
Instruments for
chaining
1) Chain or tape, 2) Arrows, 3) pegs, 4)
ranging rods,
5) offset rods, 6)
plasters laths and whiter 7)
plumb bob.
1. Chains
> Made up of straight
links of galvanized mild steel. Bent into rings at the end.
> Links are joined
together by three circular or oval shape rings.
>
The
ends of chain are provided with brass handle and length of chain is measured
from outside of one handle to outside of other handle.
> The length of the link is the distance
between the centre oftwo consecutive middle rings. Following are the commonly
used chains
a. Metric Chain
> Available in lengths
of 5,10,20 and 30 meter
> For 5m and 10m chain
tallies are fixed at every meter length and at 5m for 20m and 30m chain.
> In case of 20m and
30m chain small brass rings are provided at every meter length.
> Length of each link is
20cm
> So there are 100 and
150 links for 20m and 30m chain respectively.
b. Gunter’s Chain or
Surveyors Chain
> It is of 66ft. having
100 links, each link being 0.66ft. or 19.8cm or 7.92 inches.
> This length of chains
is adopted as it is convenient for land measurement
> 10 square chains = 1
acre = 10*162 = 43560
> 40 R = 1 acre, 100 R
= 1 hectare. = 2.5 acre.
c. Revenue Chain
33
>
It is 33feet long and has 16
links. Each link being ft
16
It is 33feet long and has 16
links. Each link being ft16
> Used for cadastral
survey.
d. Engineers Chain
> It is 100 ft. long and
has 100 links. Each link being 1 feet.
e. Steel bands or band chain
> Made up ofblue steel
available in length.
> It is of 20 and 30m
length, having links of 20cm each.
> It is accurate than
chain since its length in unalterable.
> It should be handled
carefully as it can be easily broken.
2. Tapes
Tapes are used for more accurate works and there
classification is based on material used for making it.
a.
Cloth
or linen tape: for rough
measurements lengths get altered due to stretching, likely to twist, not strong
enough easily affected due to moisture, lance may shrink.
b.
Metallic tapes: Water proof – better than cloth
tape
c.
Steel tapes: better than cloth and metallic
tape.
d.
Invar tapes:
>
Mostly
used for linear measurements when high degree of precision is required, such as
base line measurement.
> Made up of alloy of
nickel (36%) and steel.
> Low coefficient
ofthermal expansion around 1/10th ofthat steel.
> Length can be measured
rapidly and accurately.
> Invar are expensive,
softer and can deform more than steel.
3. Arrows
Common length is 40 cm – made up of good quality
hardened and tampered wire of 4 mm diameter - there should be minimum 10 arrows
with one chain.
4. Wooden pegs
Made up of timber – 3cm square and 15cm long – used to
mark positions of stations or terminal points of a survey line.
5.
Ranging rods
Length is 2 to 3mm (2m is more
common) – shod at the bottom – painted black and white or red and white. Each
bond being 20cm deep – used to range intermediate points on a survey line – 3cm
nominaldiameter.
6.
Ranging poles
Similar to ranging rods – length
4 to 8m and diameter 6 to 10cm – used in case of very long lines.
7.
Offset rods
Similar to ranging rod – has a length of3m – used for
measuring rough offsets. Butt rod – also used for measuring rough offsets –
used by building surveyors – architects.
8.
Plasters laths
Wooden 1/2 to 1m length – for ranging a live in open
ground – intermediate points can be lived out with laths
Whites: pieces of sharpened thin sticks used for same purpose
as of laths – sharpened at one end and split with the knife at top so as to
keep paper in it – temporary marking.
9.
Plumb
Bob: to transfer the
points to the ground – for centering in the theodolites, compass, plane table,
etc. – to make ranging pole vertical
Ranging a survey line
When length ofline exceeds chain
length, then some intermediate points are needed to be established in line with
two terminal stations, before chaining starts. The process of establishing or
fixing such intermediate points is known as ranging.
1.
Direct ranging
> When two ends of survey line are inter visible
> Ranging can be done with the
help of eye, or line ranger or the odolite.
> Line ranger: consist oftwo
plane mirrors or two isosceles triangles – are person can handle it and
fix intermediate points on a
survey line.
2.
Indirect or reciprocal ranging
> Adopted when both ends of
survey line are not inter visible either due to high intervening ground or
due to long distance between
them.
Errors due to
incorrect chain: When the length ofthe chain (used in measuring length)
is not equal to the
true length or designated length
ofit, then the measured length of the line with that chain will not be
correct, so necessary correction
need to be applied.
> If chain is too long, measured distance will
be lesser than the actual. Thus error will be
negative, and
correction to be applied is positive.
> If chain is too short, measured distance will
be more than the actual. Thus error will be
positive and
correction to be applied is negative.
Let,
L = True or designated length of
chain (or tape)
L’ = Incorrect or actual length
of chain (or tape) used,
l’ = Measured length of line,
l = Actual or true length of
line,
A’= Measured area or computed
area of ground,
A = Actual or true area of the
ground,
V’= Measured or computed volume,
V = Actual or true volume.
No comments:
Post a Comment