2. Definition of terms used in levelling:
A bench mark (B.M) :
-
It is fixed reference point of known elevation.
The line of collimation:
-
It is the line joining the intersection of cross hairs of the optical centre of the object glass.
It is also called the line of sight.
An axis of the telescope:
-
It is a line joining the optical center of the object glass to the center of the eye piece.
Foresight:
-
(Also called a foresight reading) It is a staff (or rod) reading on a point whose elevation is
to be determined or on a change point. It is also termed as minus sight. It is the last staff
reading denoting the shifting of the instrument.
An Intermediate sight (I.S):
-
It is any other staff reading taken on appoint of unknown elevation from the same set up
of the level. All sights taken between the back sight and the fore sight and the foresight
are intermediate sights.
A Station:
-
It is a point whose elevation is to be determined. It may be noted that it is a point where
the staff is held not the point where they leveled is set up.
3. Objective
To determine the relative heights of different objects on or below the surface of the earth
and to determine the undulation of the ground surface.
To prepare a contour map for fixing sites for reservoirs dams, barrages etc… and to fix
the alignment of roads, railways, irrigation canals, and so on.
To determine the altitudes of different important points on a hill or to know the reduce
levels of different points on or below the surface of the earth.
To prepare a longitudinal section and cross-section of a project (roads, railways,
irrigation cannel, etc) in order to determine the volume of the earth work
To prepare a layout map for the water supply, sanitary, or drainage schemes.
6. Discussion
1. At first there was a slight difference between the final RL and the initial RL,
this is due to some errors like parallax error or the uneven ground which the
equipment is set up on.
2. After that, in order to right the wrong, first we had to do the checkings by
finding the difference between the total value of backsight and the total
value of foresight, difference between the total value of rise and fall and the
difference between the final RL and the initial RL.
3. If the checkings are all the same, then we had to find the misclosure by
deducting the actual RL and the measured RL.
4. The next step is to distribute the error by the number of set ups. In this case,
there were 9 set ups. For instance,
-the first set up: (0.689÷9)x 1
-second set up :(0.689÷9)x2
-third set up
:(0.689÷9)x3
5. Since the checkings are negative, the adjustment should be added in to the
RL.
7. Conclusion
Leveling is to determine the difference in height between two points in an area.
The main purpose of leveling is to design the highways, railroad, sewers,
watersupply system and provides grade lines on the existing topography. Besides
that, leveling can also lay out the construction projects according to the planned
elevations and enables to calculate the volume of earthworks. Leveling requires
equipment such as leveling staff, tripod, auto level meter, staff bubble and
measuring tape.
Leveling method is now widely used in construction sites. Leveling is an
inexpensive, simple and accurate method for measuring height. Besides
construction site, leveling are also used for measurements of rivers or lake.
8. References
1. McCormack, AJ. "Setting out Auto-levels & Lasers." Pavingexpert.N.p., 1997. Web. 28 Nov.
2013. <http://www.pavingexpert.com/setout04.html>.
2. Wong, KhaiJee. "Levelling."Site Surveying.Taylor's Lakeside University, Subang Jaya. 11
Sept. 2013. Lecture.
3. Nosek, Aaron. "Long Tape." Make Your Mark. Keson, 1968.Web. 28 Nov. 2013.
<http://www.keson.com/products/Measuring/Long-Tapes.aspx>.
4. Retrieved from http://www.agriinfo.in/?page=topic&superid=8&topicid=54
5. Retrieved from http://en.wikipedia.org/wiki/Levelling
