Leveling and it's Types and how it work's

Leveling and it's Types and how it work's

Definition:

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Leveling is the most widely used method of obtaining the elevations of ground points relative to a reference datum and is usually carried out as a separate procedure to those used in fixing planimetric position. The basic concept of leveling involves the measurement of vertical distance relative to a horizontal line of sight. Hence it requires a graduated staff for the vertical measurements & an instrument that provides a horizontal line of sight.

Main Techinal Terms used in leveling

Bench Mark
Reduced Level
The height of any target point is referred to as Reduced Level (R L), because it is reduced to a known datum.
Back sight (BS)
Foresight (FS)
Intermediate sight (IS)

Leveling is branch of survey which deal with vertical plane's measurements the knowledge which is  relative elevations of different objects or points on the earth's surface by different instruments and machines. This is done by taking measurements in the vertical plane.

Basic Purpose of leveling

The main purpose of leveling in surveying are to find the elevations of given points with respect to a given datum. To establish points at given elevations or different elevations with respect to the given or assumed datum.In order to calculate the heights of points a datum is required, i.e. a reference level. This is usually the mean sea level. For this purpose, the use of Bench Marks is necessary, and these are classified as follows;

Bench Mark (BM) is a point with known height above mean sea level (or other reference datum) which are permanent points and are provided by the Department of Lands and Surveys. First staff reading taken after setting up the instrument. last staff reading taken before moving the instrument to another location. all readings taken between a BS and a FS.

Methods of Leveling:

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By Collimation Method 

It consist of finding the elevation of the plane of collimation ( H.I.) for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.

• Elevation of plane of collimation for the first set of the level determined by adding back side to R.L. of B.M.
• The R.L. of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (IS & FS) from the elation of the plane collimation. [H.I.]

When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding B.S. taken on the C.P. From the second position of the level to the R.L. C.P. The R.L. of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation Arithmetical check

Rise and Fall Method

Rise and Fall Method Sum of B.S. – Sum of F.S. = last R.L. – First R.L. This method is simple and easy.Reduction of levels is easy. It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between there staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.
 Arithmetic check Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L.This method is complicated and is not easy to carry out. Reduction of levels takes more time.Visualization is necessary regarding the nature of the ground. Complete check is there for all readings.This method is preferable for check levelling where number of change points are more.

Procedure
Setting up:

1.  Back sight and foresight distances should be approximately equal to avoid any errors due to collimation, refraction or earth curvature.
2.  Distances must not be so great as to not be able to read the graduations accurately.
3.  The points to be observed must be below the level of the instrument, but not lower than the height of the staff

Elimination of parallax:

Parallax is the apparent movement of the image produced by movement of the observer’s eye at the eyepiece. It is eliminated by focusing the telescope on infinity and then adjusting the eyepiece until the cross-hairs appear in sharp focus. The setting will remain constant for a particular observer’s eye.

 Booking:

• Level books or loose-leaf leveling sheets shall be numbered and indexed in a register.
• Details of the site, work, date, observer, chain-man, Booker, weather, wind, instrument and any other relevant items shall be entered.
• Enter the first observation (which is on a known point) in the Back-sight column, and sufficient detail in the Remarks column to identify it. Enter the point’s R.L. zero from the site register or plate on the BM, etc.
• Enter all other points on subsequent lines as intermediates except the point chosen as the foresight. Identify them in the Remarks column as above. Enter the foresight on a further line in the foresight column.
• Change the instrument to the next setup. Enter the following back-sight on the same line as the previous foresight but in the Back-sight column.
• Repeat the above procedure at each setup on the outward run then reverse it to work back to the starting point on the return run. The furthest point out is treated as for all other change points.

Reducing the levels:Two methods are in general use; the “rise and fall” method and the “height of collimation” method. The latter reduces levels relative to the instrument height. As it has inferior in-built checks it should not be used and will not be covered here. The “rise and fall” methods shall be used for reduction of all site leveling. Reduction shall be carried out on site before packing up to ensure that the level ling has been done correctly. 

• calculate the rises and fall between successive points and book them in the appropriate column (one can determine whether each shot is a rise or fall by the following rule of thumb: a higher value on top denotes a rise; a higher value on the bottom denotes a fall ).
• Add up the back-sight and foresight columns for the entire traverse and note the difference between them; this is the close up the rises and falls for the entire traverse, and compare the difference between them with the difference between the back-sights and foresight's; they should be the same.
• Carry the reduced levels in the R.L. column down the page by adding or subtracting the appropriate rise and fall values to the successive values of R.L. The final value of the original starting point will differ from the original value by the amount of the close.

Common sources of errors in leveling

1.  Instrument not correctly leveled.
2. Telescope not correctly focused.
3. The wrong cross-hair reading recorded (e.g. top instead of middle).
4.  Staff incorrectly read or not held vertical.
5.  Staff incorrectly booked.