Methods for Correcting the bearings affected by Local Attraction

Local Attraction | Methods for Correcting the bearings affected by Local Attraction

The deflection of a magnetic needle from its true position due to the presence of magnetic influencing material such as iron ore, magnetic rock, underground pipeline, electric cables, iron pipes, electric poles in its vicinity is called “Local Attraction”.

Methods of Correcting the bearings

There are two methods of correcting the bearings affected by local attraction:

  1. Included angle Method
  2. Error Computation

Included angle Method

In this method, the included angles of the traverse are calculated first, then starting from the line which is unaffected by local attraction and using the included angles, the corrected bearings of the traverse are computed.

Error Computation Method

In this method, the direction and the amount of local attraction at each survey station is determined.

Then starting from the line which is unaffected by local attraction, the corrected bearing of the traverse are computed.

This method is more accurate than the included angle method.

Hence it is adopted by most of the surveyors.

Read moreMethods for Correcting the bearings affected by Local Attraction

Temporary Adjustments of Prismatic Compass | Guide to Surveying

Temporary Adjustments of Prismatic Compass | Compass Surveying

In this article, we will be discussing step by step method for making Temporary adjustments in Prismatic Compass necessary for carrying out Compass Surveys.

  1. Fixing the compass to thr tripod
  2. Centering the compass
  3. Levelling the compass
  4. Sighting the object
  5. Observation of bearings

Fixing the compass to the tripod

The box of prismatic compass is fixed to a spindle of ball and socket joint. By the ball and socket arrangement, this can be quickly levelled and rotated in any direction.

Centering the compass

The prismatic compass is centered over a survey station correctly by means of a plumb bob or by dropping a pebble from the centre of the instrument.

Read moreTemporary Adjustments of Prismatic Compass | Guide to Surveying

Guide to Compass Surveying | Open and Closed Traverse

Compass Survey | Guide to Surveying and Levelling

Compass Survey is also known as Angular Surveying.

The branch of surveying in which the position of the object is located by angular measurements taken by the compass is known as “Compass Surveying”.

Purpose of Compass Surveying

When the area to be surveyed is relatively large, then the area cannot be divided into triangles.

For example:

Towns, colonies etc.

When the survey work is to be completed quickly

When the area is full of obstacles which prevent chaining.

Compass Surveying is unsuitable in areas having magnetic rock, iron core, power lines etc which attracts a magnet.

Traversing

A traverse is formed by joining the points on the ground by means of series of connected straight line.

There are two types:

  1. Open traverse
  2. Closed traverse

Read moreGuide to Compass Surveying | Open and Closed Traverse