how measure the base line in triangulation surveying


how measure the base line in triangulation surveying

upendra kumar

How to measure the baseline in triangulation surveying
The measurement of baseline has an important role in the triangulation surveying, because
the accuracy of computation of sides of the triangle depends upon the accuracy of the measured

Before measuring of baseline, a site will be chosen on the basis of the following conditions.
  1. The site should be fairly level or uniformly sloping or gently undulating.
  2. It should be free from obstructions throughout the whole of its length.
  3. The extremities of the baseline being visible at ground level.
  4. The ground should be firm and smooth.
  5. The selection of sites taken in the mind if the triangle is formed well-shaped triangles.

The equipment required to measure the baseline.

  1. Three standardized tapes
  2. Straining device.
  3. Weight and pulley
  4. Finally divided pocket scale
  5. Supporting tripods or stakes,
  6. Steel tape for setting out tripods stakes
  7. Thermometers

Procedure:- Before starting the measurement make sure that the align of the line is free from all types of
obstruction and leveled.

The line is divided into sections and aligned by transit instrument. A series of such posts
(marking or measuring posts) are accurately driven on the line with their tops about 0.6m above
the ground surface at intervals slightly less than one tape length.Supporting stakes are driven 15 to 30 meters intervals. 
how -measure- the- base- line -in- triangulation -surveying

If very accurate work use tripods instead of posts. To measure the baseline . The tape is stretched
between the marking posts or tripods and hangs freely. The rear ends of the tape are connected
to the straining posts and forward end of spring balance. In the case of tripods two tripods required
for measuring baseline one is a straining tripod and another is measuring tripods.  The proper
tensions then applied at the forward posts and measured the temperature with the help of
thermometers . Reading of the each end of tape , the exact distance between the marks on
tripod heads is determined, the reading on the scale corresponding to the mark being read with
a microscope.

Correction applied in measurement of base line
  • correction for absolute length
  • Correction for temperature
  • Correction for tension or pull
  • Correction for sag
  •  Correction for slope 
  • Reduction to sea level
The tape is then carried forward and the process repeated until the end of the section is reached.
The section is again measured in the reverse direction as a check . if the two measurement agree
within the permissible limit, 

The measurement of a long baseline may sometimes  be difficult due to obstacles or the level
ground being of limited extent, only portion of the baseline accurately measured which is then
prolonged by ranging lines in continuation of it,  at one or both ends, suitable lengths obtained
to give well- conditioned angles from its end to the points to be fixed. The lengths of the additional
Lines are calculated from the measured base and the observed angles.



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