ABSTRACT
Surveying or land surveying is a technique, profession, and science art of determination the terrestrial or three-dimensional position of points and the distance angle between them. In this experiment, levelling which is one of the branch of surveying is used to esta establ blis ish h Tempo empora rary ry Benc Bench h ar!" ar!"TB TB# # as a heig height ht refe referrence ences. s. $eve $evell llin ing g is the the measurement of geodeticheight using an optical levelling intrument and a level sta% or rod having a number scale. To &nd Temporary Bench ar!, a tripod, two sta%, two bubble sta%, auto level and measuring tape are used. Before that, a Bench ar!"B# which function as a permanent mar!ing with an establish elevation"msl#. 'fter that, all the equipment is set up for levelling purposes in a range of set perimeter for the targeted point for establishing the TB. It is desirable for TB to be something that can be found at a later time so that the elevations can be rechec!ed. INTRODUCTION
$evelling refers to height measurements for representing the relative di%erence in height "altitude# between various points on the earth(s surface. )ommon levelling instruments include the spirit level, the dumpy level, the digital level, and the laser level. In this practical session, we used traditional method of levelling where we need to be in touch on the ground. So, basic equipments li!e tripod, auto level, sta%, bubble sta%, and measuring tape are used. The purposes of doing levelling is to establish new vertical control*Be control*Bench nch ar!"B# ar!"B# and Temporary emporary Bench Bench ar!"TB# ar!"TB#.. $evellin $evelling g also is done done for determining the height of discrete points and provide spot heights or contour on a plan. +ere we can said that it is important before any construction starts to ma!e sure the land is in level and doesnt have much di%erence in gradient. If it has, then foundation has to be laid accordingly, so for this purpose levelling is carried out. In order to calculate the heights of points a datum"reference level# is required. This is usually the mean sea level. or this purpose, the use of Bench ar! is necessery, and this is classi&ed as follow Bench ar!"B# /S Bench ar!s are established by the /rdnance Survey to provide height references. They are usually carved into stonewor! or other stable material that is unli!ely to be disturbed. Temporary Temporary Bench a!"TB# ' point of !nown height aove a pre-de&ned level. This level is not absolute and is defned locally by the surveyor for the purpose of survey. Temporary Bench ar!s are often established around the survey site. TB may be surveyed into the 0atum by levelling between the site TB and an B. TB require to be stable. The main site reference is often a steel pin set in a bloc! of concrete but wooden pegs set in concrete with a nail head providing the reference level are often used.It is good practice to establish a number of TB around the perimeter of a building site as a precaution against the only site height reference being disturbed or dug up part way through the contract. The height of any any target point is referred referred to as 1educed 1educed $evel"1$# $evel"1$# because it is reduced reduced to a !nown 0atum.
Fig 1. Levelling line. For large areas a correction is required or curvature o t!e eart!.
$evel readings can be ta!en either from one location or from various stations, if some points is not visible. Basic de&nitions that should be considered are Bac!sight"BS# 2 irst sta% reading after setting up the instrument. It is a reading of the rod, ta!en when loo!ing through instrument"level# bac! toward of !nown elevation typically B and TB. oresight"S# 2 $ast sta% reading before moving to another location. It is ta!en when loo!ing through instrument forward toward a point of !nown elevation, typically TB that is establish by the surveyor. Intermediate sight"IS# 2 all readings ta!en that is not a BS and S.
If the
instrument is set up at one location only, there will 3ust one BS, S and a several IS.
)losed and open traverse 'lways commence and &nish a level run on a datum, benchmar! or !nown 1$. This is what is !nown as a closed level traverse, and will enable you to chec! the level run. )losed level traverse Series of level runs from a !nown 0atum or 1$ to a !nown 0atum or 1$. isclosure in millimeter 45 x 6!m ) 8now n
)
8nown 0atum
)losed loop level traverse Series of level runs from a !nown 0atum or 1$ bac! to the !nown 0atum or 1$. isclosure in millimeter 45 x 6!m B S9T:
S9T:
) )
S9T: S9T: TB
B
/pen level traverse Series of level runs from a !nown 0atum or 1$. This must be avoided because there no che!ed on misreading.
)
)
8now n
)
$evelling of two points is performed in two steps- S and BS. The height di%erences, S and BS should be theoratically equal. In ta!ing the reading of measurement, we need to consider the misclosure. isclosure is the amount by which the measured height di%erences"
∆ H meas
# di%ers from the !nown
∆ H KNOW N
di%erence derive from the 1$ of the star &nishing bencmar!"
IS)$/S:19 < "
#-" ∆ H KNOWN
∆ H meas
# -
Small misclosure are expeted in a closed loop due to accumulation of error. If the misclosure is small, it can be 3ust ad3usted whereas if the misclosure is large, so the processed must be repeat. isclosure also can be result from errors in published B and instability of B. The aount of misclosure depend the accuracy of what we want to achieve. To ma!e sure of it, the misclosure should be withinisclosure
≤ ±12 K
mm
=here 8 is the distance of the loop in !m or calculation, we used rise and fall method. It consists in determining the di%erences of level between consecutive points by comparing each point after the &rst with that immediately preceding it. The di%erence between their sta% reading indicates a rise or a fall according as the sta% teading at that point is smaller or greater than the preceding point. The 1$ of each point is then found by adding rise or subtractin fall to or from the 1$. The 1
method provides complete chec! on IS. >?ote
all < S- BS all < IS- BS 1ise < BS- S 1$ < @revious 1$ A 1ise 1$ < @revious 1$ 2 all
