Errors and Precautions in Tacheometric Surveying

 

Errors and Precautions in Tacheometric Surveying:

The errors may be instrumental errors, due to manipulation and sighting or due to natural causes.

The instrumental errors may be due to imperfect permanent adjustments in the instrument and due to incorrect graduations on the stadia rod.

When the instrument is in perfect permanent adjustment before starting the work, the constants of the instrument given by the manufacturer should be verified in the field by actual observation. This is very necessary for important surveys where accuracy (and not the time) is the main criterion.

The graduations on the rod should be carefully examined, and if any discrepancy is observed, suitable corrections should be applied to the observed readings.

The errors due to manipulation and sighting depend on the efficiency and skill of the surveyor. These are due to inaccurate centring and levelling of the instrument and taking incorrect Stadia readings.

To view the stadia rod clearly, there should be no Parallax. While taking stadia hair readings, care should be taken to see that the axial hair is not mistaken for stadia hair.

The accuracy of the stadia hair readings may be checked by seeing whether the mean of the stadia hair readings is equal to axial hair reading.

The errors due to natural causes may be due to wind, unequal expansion of the instrument parts, and visibility and unequal refraction. Out of these, the last one is the most important.

This happens due to unequal refraction of rays of light when they pass through layers of air of different densities. Therefore, to avoid this error, taking readings in the mid-day should be avoided as far as possible.

Also, the lines of sight should not be within a meter from the ground.

The average permissible error in the distance is 1 in 500 to 1 in 900 and in elevations 0.08 to 0.10 m.

The closing error in a tacheometry traverse should not be more than 0. l√P, where p is the perimeter of the traverse in meters.