Table of Contents
Principle Of Compass Surveying
This guide provides an overview of compass surveying, focusing on the principles of traversing, the tools involved, and their practical applications. It covers different methods like chain, compass, theodolite, and plane table traversing, and explains how to measure bearings and distances accurately using a prismatic compass. Perfect for anyone looking to understand the fundamentals of this surveying technique Principle Of Compass Surveying.
The principle of compass surveying is traversing, which consists of a series of interconnected lines. The magnetic bearings of these lines are measured using a prismatic compass, and their distances are determined using a chain. This type of survey does not necessitate the creation of a network of triangles Principle Of Compass Surveying.
Interior details are identified by taking offsets from the main survey lines. Occasionally, subsidiary lines may be used to locate these details.
Compass surveying is recommended when
- A large area to be surveyed,
- The course of a river or coastline is to be surveyed, and
- The area is congested with numerous details, making triangulation impractical.
Compass surveying is unsuitable for areas where local attraction is likely, caused by the presence of magnetic materials such as steel structures, iron-ore deposits, or electric cables carrying current, among others.
Traversing
As mentioned earlier, surveying that involves a series of interconnected lines is called traversing. The sides of the traverse are referred to as traverse legs.
In traversing, the lengths of the lines are measured using a chain, and their directions are determined using a compass, theodolite, or by forming angles with a chain and tape. A traverse can be classified into two types: closed and open, Principle Of Compass Surveying.
1. Closed Traverse
When a series of interconnected lines form a closed loop, meaning the finishing point coincides with the starting point of the survey, it is referred to as a “closed traverse.”
2. Open Traverse
When a series of connected lines extends in a general direction and does not return to the starting point, it is called an “open traverse” or “unclosed traverse.”
An open traverse is suitable for surveying features like roads, rivers, coastlines, and similar linear structures.
Methods Of Traversing
A traverse survey may be conducted by the following methods:
- Chain traversing (by chain angle),
- Compass traversing (by free needle),
- Theodolite traversing (by fast needle) and
- Plane table traversing (by plane table).
1. Chain Traversing
Chain traversing is primarily carried out when triangulation cannot be adopted. In this method, the angles between adjacent sides are determined by chain angles. The entire survey relies solely on the use of a chain and tape, with no angular measurements involved. Chain traversing is typically employed in situations where forming triangles is not feasible, such as surveying a pond, Principle Of Compass Surveying.
2. Compass Traversing
In this method, the fore bearings and back bearings of the traverse legs are measured using a prismatic compass, while the lengths of the traverse sides are measured with a chain or tape. The observed bearings are then verified, and necessary corrections for local attraction are applied.
When the traverse is plotted, a closing error may occur. This error is adjusted graphically using Bowditch’s rule.
3. Theodolite Traversing
In this type of traversing, the horizontal angles between the traverse legs are measured using a theodolite, while the lengths of the legs are determined using a chain or by applying the stadia method. The magnetic bearing of the starting leg is measured with the theodolite, and the magnetic bearings of the remaining sides are subsequently calculated, Principle Of Compass Surveying.
The independent coordinates of all the traverse stations are then determined. This method is highly accurate.
4. Plane Table Traversing
In this method, a plane table is set up at each traverse station, progressing in either a clockwise or anticlockwise direction, and the circuit is eventually closed. During the traversing process, the sides of the traverse are plotted to a suitable scale. At the completion of the survey, any closing error that arises is adjusted graphically, Principle Of Compass Surveying.
Types Of Compasses
There are two types of compasses:
- The prismatic compass, and
- The surveyor’s compass.
1. Prismatic Compass
In this type of compass, the readings are obtained using a prism.
The essential components of this compass are as follows:
(a) Compass Box
The compass box is a circular metallic container, typically with a diameter of 8 to 10 cm, and made from non-magnetic material. A pivot with a sharp point is positioned at the center of the box.
(b) Magnetic Needle and Graduated Ring
The magnetic needle is made of a broad, magnetized iron bar, with pointed ends. The needle is mounted on a graduated aluminum ring, which allows for precise reading of the bearings.
The graduated ring is marked from 0Β° to 360Β° clockwise, with the graduations starting at the south end of the needle. Thus, 0Β° is at the south, 90Β° at the west, 180Β° at the north, and 270Β° at the east. The degrees are further subdivided into half-degrees, and the figures are written upside down Principle Of Compass Surveying.
The arrangement of the needle and ring includes an agate cap pivoted on the central pivot point. Additionally, a rider made of brass or silver coil is attached to the needle to counterbalance its dip.
(c) Sight Vane and Prism
Β The sight vane and the reflecting prism are fixed diametrically opposite each other on the compass box. The sight vane is hinged to the metal box and features a horsehair at its center for precise alignment. The prism includes a sighting slit at the top, as well as two small circular holesβone at the bottom of the prism and another at the side for the observer’s eye, Principle Of Compass Surveying.
(d) Dark Glasses
Two dark glasses are provided with the prism. The red glass is used for sighting luminous objects at night, while the blue glass helps reduce eye strain for the observer in bright daylight.
(e) Adjustable Mirror
A mirror is attached to the sight vane, and it can be lowered, raised, or inclined as needed. This allows for the adjustment of the line of sight when an object is too low or too high. The mirror enables the observer to view the object through reflection.
(f) Brake Pin
A brake pin is located at the base of the sight vane. When gently pressed, it halts the oscillations of the ring, providing stability for accurate readings.
(g) Lifting Pin
A lifting pin is situated just below the sight vane. When the sight vane is folded, it presses the lifting pin, which in turn lifts the magnetic needle off the pivot point, preventing any potential damage to the pivot head.
(h) Glass Cover
A glass cover is placed on top of the box to protect the aluminum ring from dust and other debris, ensuring the accuracy and longevity of the instrument.
Temporary Adjustment Of Prismatic Compass (Field Procedure Of Observing Bearing)
The following procedure should be adopted while measuring the bearing by prismatic compass.
1. Fixing the Compass with Tripod Stand
The tripod stand is positioned at the required station with its legs spread wide for stability. The prismatic compass is held with the left hand and placed onto the threaded top of the stand. Next, the compass box is rotated clockwise with the right hand, securing the threaded base of the compass box to the threaded top of the stand, ensuring the compass is securely mounted, Principle Of Compass Surveying.
2. Centering
Typically, the compass is centered by dropping a small piece of stone from the bottom of the compass box. Alternatively, centering can be achieved using a plumb bob, which is held centrally below the compass box to ensure precise alignment.
3. Levelling
Leveling is accomplished using a ball-and-socket arrangement on top of the tripod stand. This arrangement is loosened, allowing the compass box to be adjusted. The box is positioned so that the graduated ring can rotate freely without coming into contact with the bottom of the box or the glass cover on top, ensuring smooth operation and accurate readings, Principle Of Compass Surveying.
4. Adjustment of Prism
The prism is adjusted by moving it up and down until the figures on the graduated ring are viewed sharply and clearly through the prism, ensuring that the reading is precise and legible.
5. Observation of Bearing
After centering and leveling the compass box over the station, the ranging rod at the required station is bisected accurately by sighting through the slit of the prism and the horsehair on the sight vane. At this point, the graduated ring may rotate quickly. The brake pin is gently pressed to stop this rotation. Once the ring comes to rest, the box is lightly tapped to verify the horizontality of the ring and the effect of friction on the pivot point. Then, the reading is taken from the graduated ring through the hole in the prism. This reading represents the magnetic bearing of the line, Principle Of Compass Surveying.
Understanding Magnetic Bearings Surveying
This guide covers essential surveying techniques, focusing on traversing, magnetic bearings, and meridians.Β Understanding Magnetic Bearings SurveyingΒ It explains concepts like true, magnetic, arbitrary, and grid meridians, as well as methods to handle magnetic declination, local attraction, and error corrections in survey readings. Ideal for understanding the foundational principles of compass traversing and surveying accuracy.
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