In surveying, a baseline is a line between two points on the earth’s surface and the direction and
distance between them. In triangulation, the base line is the most important line and is measured
very precisely. This is so because the accuracy of triangulation depends upon the accuracy of the
sides of the triangles computed from the base line. In addition to being a supplement to angular
measurement in triangulation, the base line measurement may also be used as a base to determine
the scale of triangulation
Site Selection for Base Line:
In selecting site for a base line, the following requirement should be considered.
- The site should be fairly leveled or uniformly sloping.
- Should be free from obstructions throughout the entire length.
- Ground should be firmed and smooth.
- The site should be such that well shaped triangle can be obtained in connecting the end stations of the base line to the main triangulation stations.
- It is preferable to choose a base as small as possible so as to measure it precisely.
This method was introduced by Jaderin and has become very popular due to following reasons:
- It is less expensive and the speed of measurement is high.
- Due to longer lengths of the tape, the choice of the base sites becomes wider.
Instruments for Baseline Measurement:
- Steel (Invar) Tape
- Auto level
- Spring balance or weight
- Leveling staff
- Plumb bob
- Designated Length or Nominal Length is indicated length as given by the manufacturer (without correction)
- Absolute Length is the true length between the two points.
Rule of Thumb:
- If the actual length of the tape (that you are using) is too long, the measured distance will be too less and the correction will always be positive and if the actual length of the tape (that you are using) is too short, the measured distance will be too long and the correction will always be negative.
- Two points are taken A and D have some known distance between these two points.
- Place the Theodolite on one of the station and ranging rod on the other station.
- Placed the two or three tripod stand between the two measuring points A and D.
- Alignment (ranging) of all the tripods is carried out along line AD.
- After alignment, tripods are placed at station A and D.
- The tripods are placed at a spacing (equal or random) depending upon the length of the base line.
- The temperature is noted down at each point for temperature correction.
- During length measurement, tension (pull) is produced in the tape which is measured by the spring balance.
- To find the accurate length, put one end of tape on one tripod stand and other end on second tripod stand.
- Load is applied on one end of tape when tension is produced in the tape and it is measured by the spring balance.
- Somewhere in the field another theodolite is placed (if the distance is greater); it is used to keep the tripod heads at the same level.
- Difference between the heads of tripods is calculated with the help of auto level by taking
different staff readings.
- During this procedure, correction for slope is adopted.
- Total base length = lengths between the individual tripods (A to B, B to C and C to D).
- Different tape corrections are applied to obtain the exact length of base line.
Corrections in Taping:
- A correction is said to be plus or positive when the uncorrected length is required to be increased in order to obtain the true length.
- A correction is said to be negative when the uncorrected length is required to be decreased in order to obtain the true length
Correction for Absolute Length
Absolute length = Nominal Length ± Ca
Ca = L (l’ – l)/L
L = Measured or Recorded Length
|l||=||Nominal Length of a tape (say 30m)|
|l’||=||Actual Length of a tape (say 30.011m)|
|Ca||=||Correction for Absolute Length|
Note: The sign of the correction depends on the values of l & l’. However, since l’ is usually greater than l, this correction is usually positive and shows that when a tape is stretched and is too long, it reads too short.
Correction for Temperature
Ct = α(Tm-To)L
|α||=||Coefficient of Thermal Expansion (e.g.|
|0.0000112 per °C for steel)|
|=||= Mean Temperature during Measurement
= Temperature of calibration Measured Length
Correction for Pull/Tension
Cp = (P – Po)L/AE
|P||=||Pull applied during measurement (usually 50N)|
|Po||=||Pull under which the tape was calibrated|
|A||=||Cross sectional Area of the tape|
|E||=||Modulus of Elasticity of tape material (for steel
tapes, typically 200,000 N mm-2 or 200GPa)
Correction for Sag
Cs = w2 L3 cos2φ/(24P²)
Cs = W2 L cos2φ/(24P²)
|P||=||Pull applied during measurement|
|φ||=||Angle of slope between tape supports|
|w||=||Weight of tape per unit length|
|W||=||Total weight of tape|
Note: When converting a measurement with tape suspended in catenary (sag) to its equivalent chord length, the correction is always negative
Correction for Slope
C = L (1 – cosφ )
C = L – √(L² – h²)
C = h²/(2 L)
Note: When converting an observed slope distance to its horizontal equivalent, this correction is always negative and applied to the measured length