Chapter: Civil Surveying : Fundamentals and Chain Surveying
Surveying Corrections for Baseline
Corrections.
for Baseline
It is necessary to
apply the following corrections to the field measurements of base line order
its true length:
1.Correction for
absolute length Adverti 2.Correction for temperature
3.Correction of tension
or pull
4.Correction for Sag
5.Correction for slope
of vertica
It may be noted that
each section of a base line is separately corrected.
1. Baseline Correction
for Absolute Length:
It is given by the
formula
Where CA = Correction
for absolute length L = measured length
of base l = Nominal length of measuring
unit
C = Correction to measuring unit
Sign of CA is the same
as that of C
Nominal length: The
designated length i.e 50 tape, 100 tape (30 m tape)
Absolute length: The
actual length under specified conditions
2. Correction for
Temperature:
It is given by the
formula
Ct = Inf (Tm-To) --
> (B)
Where
Ct = Correction for
temp
= co-efficient of thermal
Tm = Mean Temperature
during
To = Temp at which the
measuring is standardized
Steel = 0.0000099-0.000012/c
Steel = 0.0000055 - 0.0000070/ F
The sign of 'Ct' is
plus or minus according to as 'Tm' is greater or less then 'to'
1. Correction
for Pull or Tension
Cp
= (Pm - Po)/AE . L -- > (c)
Where
CP
= Correction for pull
Pm
= Pull applied during measurement
Po
= Pull at which the measurement unit (tape) is standardized
L
= length measured
A
= Cross-Sectional area of measuring unit
E
= Modulus of elasticity of measuring unit
E
steel = 21 x 105 kg / cm2
E
steel = 30 x 106 /bs/in2
The
sing of this correction is always plus (T) as the effect of pull is to increase
the length of the tape and consequently to decrease the measured length of the
base.
4.
Correction for Sag:
Correction
for sag is the deference in length between the are and its chord i.e b/t the
curved length of the laps and the distance between the supports. It's us
required only when the tape is
suspended
during measurement. Since the effect of sag is to make the measured length too
large, it is always subtractive.
It
is given by the formula.
Cs=
L1(WL12)/24Pm2 -- > (D)
Where
Cs = Correction for Sag
L1
= Distance b/t supports.
W
= wt of tape per unit length
Pm
= applied pull
W
= wt of tape b/t supports.
If
there are 'n' equal space per tape length,
L
= nL1
Sag
correction per tape length is
Cs
= nL1(WL1)2/24Pm2 = L(WL1)2/24Pm2
= L(WL)2/24nPm2
Total sag correction to
measures length
Cs=N x Cs/ + Cs//
Where N = no of whole
tape length
Cs/ = Sag correction
per tape length.
Cs// = Sag correction
for any tape length
Normal tension:
The normal tension of a
tape is a tension which will cause the effects of pull and sag to neutralize
each other. It may be obtained by equating the corrections for pull and sag
Pn = 0.0204W Rt(AE) /
Rt(Pn-Po) -- > (E)
Where Pn = normal
tension
W = wt of tape b/w
supports
Pn is determine by
trial
5. Correction for Slope
or Vertical Alignment:
This correction is
required when the points of supports are not exactly at the same level
L1, L2------------
= Successive length of
uniform garages
B1, b2------------ = Difference of elevation b/t the
extremities of each of these grades.
Cg = corr for slope
Cg = B1C1 = AC1 - AB1 =
l -D
Cg =l-Rt(l2-h2) -- >
Exact
This correction is
always negative for measured length.
Related Topics