Advantages are 1. Possibility of omitting measurement is eliminated. 2. The surveyor can compare the plotted work in the field then and there only. 3. Irregular objects are plotted more accurately, since they are seen while plotting. 4. Booking errors are eliminated. 5. Local attractions do not influence the plotting. 6. No great skill is…
The errors may be grouped into the instrumental and personal errors. Instrumental Errors 1. The surface of plane table not perfectly plane. 2. Bevelled edge of alidade not straight. 3. Sight vanes of alidade not perfectly perpendicular to the base. 4. Plane table clamp being loose. 5. Magnetic compass being sluggish. 6. Drawing sheet being…
The following four methods are available for carrying out plane table survey: 1. Radiation 2. Intersection 3. Traversing 4. Resection. The first two methods are employed for locating details while the other two methods are used for locating position of plane table station on drawing sheet. Radiation After setting the plane table on a station,…
The most commonly used plane table It consists of a well-seasoned wooden table Top mounted on a tripod. The table top can rotate about vertical axis freely. Whenever necessary table can be clamped in the desired orientation. The table can be levelled by adjusting tripod legs. The following accessories are required to carry out plane table survey:…
A freely suspended and properly balanced magnetic needle is expected to show magnetic meridian.However, local objects like electric wires and objects of steel attract magnetic needle towards themselves.Thus, needle is forced to show slightly different direction. This disturbance is called local attraction. The list of materials which cause local attraction are: (i) magnetic rock or iron ore, (ii) steel structures, iron…
The magnetic meridian and the true meridian may not coincide with each other in a place. The horizontal angle between these two meridians is known as magnetic declination. The magnetic north at a place may be towards east or west of true north . If it is towards east, it is known as eastern or +ve declination. Western…
As stated earlier a bearing of a line is the angle made by the line with respect to a reference direction, the reference direction being known as meridian. The direction shown by a freely suspended and properly balanced magnetic needle is called magnetic meridian and the horizontal angle made by a line with This meridian…
To take a reading from a compass, the following temporary adjustments are required: (i) Centring: The compass should be fixed to the stand and set over the station. To centre the compass legs of the tripod stand should be moved inward-outward or in a circumferential direction. To check centring plumb may be used or a pebble…
The types of compass that are used commonly are: (i) prismatic compass; and (ii) surveyor compass. The essential parts of both type are: (i) A magnetic needle, (ii) A graduated circle, (iii) A line of sight, and (iv) A box to house them. There are some differences in the essential parts of the two type of…
Disadvantage of chain surveying is that, in it only distances are measured and hence area is to be covered with a network of triangles. If the length as well as angle of a line can be measured with respect to a known direction then it is possible to plot a line, independent of length of…