COMPASS SURVEYING — Purpose, Principles, Field Procedure, Errors And Precautions

In chain surveying the area to be surveyed is divided into connected triangles and only linear measurements are taken The chain surveying is suitable when the area to be surveyed is small and fairly flat. However, when the area to be surveyed is large, then chain surveying alone is not sufficient, the direction and angles are also to be measured. For measuring the direction and angular measurements the following instruments are used.
(a) Measurement of direction
(i) Prismatic compass
(ii) Surveyor’s compass
(b) Angular measurements.
(i) Theodolite
(ii) Box sextant

PURPOSE OF COMPASS SURVEYING

1. When the area to be surveyed is large.
2. When the area is undulated and chaining is difficult.
3. When the area cannot be divided into triangles eg: towns, urban areas etc.
4. When the surveying is done in dense forest.
5. When the surveying is to be completed quickly.
6.When the area to be surveyed is long and narrow, eg Roads Railway lines, Streams etc.
Note: Compass surveying is not recommended for areas where local attraction is suspected due to the presence of steel structures, iron ore deposits, electrical cables etc.

PRINCIPLE OF COMPASS SURVEYING

The principle of compass surveying is traversing. It involves a series of connected lines. In traversing it is not required to form a network of triangles. In compass traversing the direction of lines are measured by prismatic or surveyor’s compass and the distances are measured by chain or tape. The interior details are located either by taking offsets from the main survey lines or by running subsidiary lines.

INSTRUMENTS USED IN COMPASS SURVEY

The following instruments are used while conducting compass survey.
1. Prismatic compass with tripod.
2. Metric chain 20 or 30 m
3. Metallic tape 20 or 30 m
4. Arrows
5. Ranging rods
6. Plumb-bob
7. Cross-staff
8. Wooden pegs and mallet
9. Tri-square or wooden set square.
10. Field book, pencil, eraser, knife etc

FIELD PROCEDURE OF COMPASS SURVEYING

Compass surveying is conducted in occordance with the following steps:
1. Reconnaisance:
To select traverse stations, the area to be surveyed is throughly examined. The stations should be selected in such a manner that, they are intervisible and covers the whole area. The
traverse station should be free from magnetic substances. The traverse legs should run along fairly level ground.
2. Preparation of Index Sketch :
After reconnaissance, an index sketch should be prepared showing the skeleton of the traverse and details of the existing features like buildings, roads etc.
3. Marking Station on The Ground:
The traverse stations are marked on the ground by wooden pegs. The pegs should be fixed on the station points in such a way that, they are exposed a few centimeters above the ground surface.
4. Preparation of Reference Sketches for Stations:
Reference sketches should be prepared for all the traverse stations by taking at least two measurements from important points. This precaution should be taken to avoid the removal of pegs or dislocation
5. Measurement of Bearings of Traverse Legs : The traverse may run either clockwise or anticlockwise however the direction should be mention in the index sketch. Refering to, suppose five traverse stations A, B, C, D and E are selected to enclose an area, in anti- clockwise direction. The prismatic compass is centred and levelled over station A. The fore bearing of AB and back bearing of EA are taken from this station. The distance AB is measured and offsets are taken along the line AB and recorded in the field note book.The compass is then shifted and centered over station B From here, the FB of BC and B.B of AB are taken. The FB and B.B of line line AB should differ exactly by 180°. If there is any error, check the centering on station B and should be adjusted properly. Now, the length of line BC should be measured. Similarly, all the traverse legs are measured and noted in the field book.
After the completion of field work, the observations are tabulated and necessary corrections are applied for local attraction,

ERRORS IN COMPASS SURVEYING

The following are types of errors which may occur while taking readings with a compass.
(a) Instrumental Errors:
1. The pivot being bent or the pivot point may be blunt.
2. The magnetic needle may not being perfectly straight and balanced.
3. The horse hair in object vane may not be straight and vertical.
4. Inaccurate graduations on the ring.
5. Plane of sight not being vertical.
6. Due to dust formation over the prism, the graduations on the ring may not be visible clearly.
(b) Personal Errors :
1. In accurate centering of compass over station.
2. Inaccurate leveling of the compass box.
3. Imperfect bisection of the object.
4. Carelessness in reading and entering the observations.
5. Carrying magnetic substances by the observer.
c) Natural Errors:
1. Local attraction due to the presence of magnetic substances near by to the instrument station.
2. Variations in declination, Le., secular, annual and diurnal.
3. Irregular variations due to natural causes.
4. Magnetic changes in the atmosphere due to the clouds and storms.

PRECAUTIONS TO BE TAKEN IN COMPASS SURVEYING

The following precautions should be taken, to avoid error while conducting a compass surveying.
1. The centering and levelling should be done perfectly.
2. The instrument stations should not be selected near magnetic substances.
3. Care should be taken by the observer not to carry any magnetic substances.
4. Readings should be observed simultaneously when sighting is done, to avoid error in measuring the readings.
5. To stop the rotation of the graduated ring, apply the break pin very gently and not suddenly.
6. To avoid wear and tear of pivot, close the object vane while shifting the instrument from one station to other.
7. Struck the compass box gently, before taking the reading, this is done to check whether the needle rotates freely or not.
8. Check the instrument before going to the field work.
9. Avoid compass surveying in adverse climatic conditions. i.e., too hot, cloudy etc.