Home | Engineering | Surveying I - CE6304 | Surveying Corrections for Baseline

Civil Surveying - Fundamentals and Chain Surveying

Surveying Corrections for Baseline

   Posted On :  21.08.2016 12:10 pm

It is necessary to apply the following corrections to the field measurements of base line order its true length:

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)





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)



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.


�        If grades are given in terms in terms o

Tags : Civil Surveying - Fundamentals and Chain Surveying
Last 30 days 166 views