1  surveying II

Magnetic compass: The most common type of compass is the magnetic compass, which is used to determine the direction of magnetic north. A magnetic compass is made by placing a bit of magnetized iron or steel which is set in a low friction so that it is allowed to move freely. In most compasses, the north end of the metal piece is marked with red paint so that all directions may be determined.

    Gyro compass: It is a special type of compass developed in 19th century which determines the true north. A gyro compass is basically a very fast spinning wheel or ball which uses the law of conservation of angular momentum and the spinning of earth's axis to point towards the true north. The gyro compass is commonly used in large ships and in other circumstances where the accurate prediction of true north is needed.

    Astrocompass: This is another type of compass which can predict true north rather than magnetic north. This compass relies on the direction of celestial bodies to find true north, which is used in many circumstances, mainly in the far north and south poles where magnetic compass would become erratic and gyro compass stops working. The use of astro compass in determining the exact direction of true north requires the accurate information of time, date, longitudinal and latitudinal location.

    Solid state compass: In this modern digital world, solid state compass are becoming more popular. These use many of the electronic magnetic sensors which would calculate the accurate direction the compass is pointing.

    GPS compass: GPS compasses are rapidly replacing the use of other traditional compasses. However, most military and ships use the gyro compass or magnetic compass if GPS compass could not pick up enough satellite. GPS compasses make use of satellites in a geo synchronous orbit over the earth to distinguish the bearer's exact location and direction they are heading. Many hikers and drivers like this compass due to its relative reliability.

    Base plate compass: It is the most affordable compass. The liquid filled compass rise on the rectangular base made of clear plastic. This compass includes a magnifying lens for map reading, luminous components for low light conditions and different scales for world wide use. This compass is fine for plotting purposes.

    Card compass: Card compass or marine compass is commonly used in ships and boats. It uses a fixed needle, depending on the moving compass card for directional readings since the moving card absorbs much of the motion of the boat.

    Thumb compass: This compass attaches to the user's thumb allowing the user to hold both the map and the compass in one hand while traveling at speed by bike or by canoe. It is also known as competition compass.

    Prismatic compass: It is a sophisticated device designed for highly accurate navigation. The prism sighting arrangement allows the user to read the compass bearings while seeing distant objects.

Q-5:   Methods of Plane Tabling

1. Radiation:

In this position the point is located on plane by drawing a ray from the plane table station to the point, and plotting scale along the ray the distance is measured from the station to the point. The method is suitable for the survey of the small areas which can be commanded from a single station.

2. Intersection:

In this method the point is fixed on plane by the intersection of the rays drawn from the two instrument stations. The line joining at these stations is called the base line.

3. Traversing:

This method is similar to that of compass or transit traversing. It is used for running survey lines between stations which have been previously fixed by other methods of surveying to locate the topographical details. It is also suitable for the survey of roads, rivers etc.

4. Resection:

This method is used for establishing the instrument stations only. After fixing the station, details are located either by radiation or intersection. The characteristic feature of resection is that the point plotted on plan is the station occupied by the plane table.

Radiation:

    Select appoint p so that all point to be located are visible from it.

    Set up the table at p and leveled.

    Marked the direction of magnetic meridian with through compass in top corner of sheet.

    Select the point p on the sheet so that it is exactly over station p on the ground.

    Centering the Alidades on p, sight the various points A, B, C etc. and draw the rays along edge of Alidades.

   Measure the distance PA, PB, PC etc. from p to various point with chain or tape and plot them to scale along the co responding rays. Join the points a, b, c, etc. to give outline of survey.