Compass surveying is the branch of surveying in which the position of an object is located using angular measurements determined by a compass and linear measurements using a chain or tape. Compass surveying is used in following circumstances:
- If the surveying area is large, chain surveying is not adopted for surveying rather compass surveying is employed.
- If the plot for surveying has numerous obstacles and undulations which prevents chaining.
- when there is a time limit for surveying, compass surveying is usually adopted
Compass surveying can be defined as the method of surveying in which the directions of the survey lines is determined by means of a compass and also by using tape or a chain. This measurement is directly done on the surface of the earth.
Even after different findings in the field of surveying, this method of compass surveying is still used for preliminary measurements in engineering surveying. However, Most of the land surveying is conducted by compass surveying by the civil engineers and surveyors which helps the foresters, geologists etc.
Initially, the theodolites where equipped with a compass which made the same heavy. But new theodolite does not have a compass in it.
Also, the compass surveying method make use of a compass to carry out the angular measurements.
The magnetic meridian is the basis on which the magnetic compass measures the directions. However, this will require the magnetized needle, a line of sight and a graduated circle.
Principle of Compass Surveying
The main principle of compass surveying is traversing. Traverse, a series of connected lines is measures by a prismatic compass. Compass surveying is mainly employed in areas that is free from local attraction. Therefore, this can be used for surveying the land area with undulations which is crowded and large.
Advantages of Compass surveying
- They are portable and light weight.
- They have fewer settings to fix it on a station
- The error in direction produced in a single survey line does not affect other lines.
- It is suitable to retrace old surveys
Disadvantages of Compass surveying
- It is less precise compared to other advanced methods of surveying.
- It is easily subjected to various errors such as errors adjoining to magnetic meridian, local attraction etc.
- Imperfect sighting of the ranging rods and inaccurate leveling also causes error.
Errors in compass survey
- Instrumental errors
- Personal errors
- Natural errors
As the name suggests they are arise due to the wrong adjustments of the instruments. Some other reasons are:
- If the plane of sight not being vertical, it also causes error in sighting and reading.
- If the magnetic needle is not perfectly straight or if it is sluggish, readings may not be accurate.
They arise mainly due to the carelessness of the surveyor. They are:
- Inaccurate leveling
- Improper reading
- Inaccurate centering
Natural errors are occurring due to the various natural causes which affect the working of compass. It has nothing to do with the surveyor and to minimize them, some corrections in calculations applied. They are:
- Local attraction
- Proximity to the magnetic storms
Types of Compass
- Prismatic Compass
- Surveyor’s Compass
- Transit compass
Prismatic compass is a portable magnetic compass which can be either used as a hand instrument or can be fitted on a tripod. It contains a prism which is used for accurate measurement of readings. The greatest advantage of this compass is both sighting and reading can be done simultaneously without changing the position. Least count means the minimum value that an instrument can read which is 30 minutes in case of prismatic compass. It means compass can read only those observations which are multiples of 30 minutes. For example- 5° 30′, 16° 00′, 35° 30′
The essential parts of the prismatic compass are listed below
- Magnetic needle
- Eye vane
- Cross hair
- Eye slit
- Eye hole
- Mirror (adjustable)
- Glass cover
- Graduated ring
- Lifting lever
- Lifting pin
- Metal box (85–100 mm diameter)
- Focusing stud
- Object vane
- Sun glasses
- Sliding arrangement for mirror
Adjustments of prismatic compass
Two types of adjustments
- Temporary adjustment
- Permanent adjustment
- Centering: it is the process of fixing the compass exactly over the station. Centering is usually done by adjusting the tripod legs. Also a plumb-bob is used to judge the accurate centering of instruments over the station.
- Leveling: the instrument has to be leveled if it is used as in hand or mounted over a tripod. If it is used as in hand, the graduated disc should swing freely and appears to be completely level in reference to the top edge of the case. However, If the tripod is used, they usually have a ball and socket arrangement for leveling purpose.
- Focusing the prism: Prism can be slide up or down for focusing to make the readings clear and readable.
They can be done only in the circumstances where the internal parts of the prism is disturbed or damaged. They are:
- Adjustments in levels
- Adjustment of pivot point
- Adjustments of sight vanes
- Adjustment of needle
Surveyor’s compass is an instrument used in surveying to measure horizontal angles. The surveyor’s compass consists of a circular brass box containing a magnetic needle which swings freely over a brass circle which divides into 360 degrees. The horizontal angle is generally measured using a pair of sights located on north – south axis of the compass. They are usually mounted over a tripod and leveled using a ball and socket mechanism.