INTRODUCTION
The definition of levelling is measurement process to determine the difference between high points on the earth referred to the datum level by using staff. Surveying based on measuring two quantities, height and distance. Measured of the linear distance horizontal and vertical distance. Levelling is the procedure for determining differences in elevation points that are some distance from each other. An elevation is the vertical distance above or below a reference datum. Elevation can be determined using the levelling techniques. Vertical distance measurement is called “levelling” includes procedures that determine.
OBJECTIVE
To get the different height between two point.
Built Bench Mark and Temporary Bench Mark for several construction projects.
To get the cross-section and longitudinal section to show the profile of soil especially for passage project. With that all the preparation will be done.
To produce contour map for a location of construction.
For works of setting out.
Mark the slope of location for the several reasons such as a stream of water and other else.
THEORY
Levelling or leveling is the measurement of geodetic height using a levelling instrument and a level staff.Difference levelling is the process used to determine a difference in elevation between two points. A level is an instrument with a telescope that can be levelled with a spirit bubble. The optical line of sight forms a horizontal plane, which is at the same elevation as the telescope crosshair. By reading a graduated rod held vertically on a point of known elevation (Bench Mark) a
difference in elevation can be measured and a height of instrument (H.I) calculated by adding the rod reading to the elevation of the BM. Once the height of instrument is established, rod readings can be taken on subsequent points and their elevations calculated by simple subtracting the reading from the height. The first reading from any instrument position is termed back sight (BS). Readings which are neither the first nor the last from the instrument station are termed intermediates (IS). The last reading from an instrument position is termed a foresight (FS).The concept of levelling based on the horizontal line through the level with level line through the points on the surface of the earth.
Example diagram measuring levelling position:
INSTRUMENT
Equipments levelling (two peg tests): Level and tripod (1 set)
Staff (2 sets)
Staff bubbles (2 sets)
Equipments levelling (establish temporary benchmark (TBM)) Level and tripod (1 set)
Staff (2 sets)
Staff bubbles (2 sets)
Fundamental two types of equipment:
(i) Level
(ii) Levelling rod
Types of level:
(i) Dumpy & Wye levels (ii) Tilting levels
(iii) Automatic level (usual used) (iv) Digital levels
(v) Electronic laser levels
Levelling rod/levelling staff:
Levelling rod is manufactured from wood, metal, or fibreglass and is
graduated in meters or feet. The metric rod can usually be directly only to 0.01 m.
FIELD PROCEDURES
Surveying work must be started from a Bench Mark (BM @ TBM).
All the observation must started from the back sight observation to fore sight observation. The distance between back sight view and fore sight view must same to overcome the collimation error.
a. Observation procedures
A series of measurements of level loop considered as shown in figure 1
Started from BM A, the first reading measurement from Setup 1 is a back sight to BM A, and the second reading is a fore sight to CP 1known as Change Point.
The levelling staff then remains at CP1, while the instrument moves to Setup 2. Take back sight reading is made to CP1, and fore sight reading to CP2, and so on.
Eventually, a Setup 4, a back sight reading is made to CP3, and foresight reading to BM A.
The traverse is then CLOSED. This allows a check on the levels, since the sum of all the changes in level should be zero.
b. Booking Procedures
The Rise and Fall Method
The terms that are commonly used in the Rise and Fall Method are: BS=Backsight
FS=Foresight
IS=Intermediate Sight RL=Reduced Level
c. Reduction (Calculation) Procedures
Survey accuracy should be checked, and should be within tolerances given below or suitable for the project. This check should be applied before the job was leaved.
The arithmetic check of the level reductions must be exact: Sum of BS-sum of FS = sum of Rise- sum of Fall (BS)-(FS) = RL(last BM)-RL(Start BM)
Misclosure should be:
ANALYSIS
Levelling calculation (rise and fall method)
Difference of height between two points: ∆ h = BS - FS
If the difference (+∆h) = rise If the difference (-∆h) = fall
BS FS ∆h Rise Fall Remarks
1.441 BM1=2.014 1.124 1.309 1.441-1.309 =+0.132(rise) 0.132 CP1 1.251 1.235 1.124-1.235 =-0.111(fall) 0.111 CP2 1.300 1.256 1.251-1.256 =-0.005(fall) 0.005 CP3 1.246 1.254 1.300-1.254 =+0.046(rise) 0.046 CP4 1.271 1.296 1.246-1.296 =-0.005(fall) 0.005 CP5 1.198 1.088 1.271-1.088 =+0.183(rise) 0.183 CP6 1.320 1.326 1.198-1.326 =-0.128(fall) 0.128 TBM 1.272 1.323 1.320-1.323 =-0.003(fall) 0.003 CP7 1.320 1.279 1.272-1.279 =-0.007(fall) 0.007 CP8 1.316 1.280 1.320-1.280 =+0.040(rise) 0.040 CP9 1.389 1.339 1.316-1.339 =-0.023(fall) 0.023 CP10 1.301 1.305 1.389-1.305 =+0.084(rise) 0.084 CP11 1.333 1.301 1.301-1.301 =+0.000(rise) 0.000 CP12 1.489 1.333-1.489 =-0.156(fall) 0.156 BM=2.014
Reduced level
Calculated as follows:
Reduced level = reduced level before + ∆h (rise) or
Reduced level = reduced level before - ∆h (fall)
Rise Fall Reduced level Remarks
2.014 BM1=2.014 0.132 2.014+0.132=2.146 CP1 0.111 2.146-0.111=2.035 CP2 0.005 2.035-0.005=2.030 CP3 0.046 2.030+0.046=2.076 CP4 0.050 2.076-0.050=2.026 CP5 0.183 2.026+0.183=2.209 CP6 0.128 2.209-0.128=2.081 TBM 0.003 2.081-0.003=2.078 CP7 0.007 2.078-0.007=2.071 CP8 0.040 2.071+0.040=2.111 CP9 0.023 2.111-0.023=2.088 CP10 0.084 2.088+0.084=2.172 CP11 0.000 2.172+0.000=2.172 CP12 0.156 2.172-0.156=2.016 BM1=2.014
Arithmetic check ∑ BS = 1.441+1.124+1.251+1.300+1.246+1.271+1.198+1.320+1.272+1.320+1.316 +1.389+1.301+1.333 =18.082 ∑ FS = 1.309+1.235+1.256+1.254+1.296+1.088+1.326+1.323+1.279+1.280+1.339 +1.305+1.301+1.489 =18.080 ∑ Rise = 0.132+0.046+0.183+0.040+0.084+0.000 =0.485 ∑ Fall = 0.111+0.005+0.050+0.218+0.003+0.007+0.023+0.156 =0.483 Therefore :
∑ BS - ∑ FS = ∑ Rise - ∑ Fall = Reduced level of last point – reduced level of first point 18.082 – 18.080 = 0.485 – 0.483 = 2.016 – 2.014
0.002 = 0.002 =0.002
True error
= 2.016 - 2.014 = 0.002m @ 2mm
*Arithmetic error in positive sign(+ve) correction must be negative sign(-ve) Arithmetic error in negative sign(-ve) correction must be positive sign (+ve)
Permissible error *D +
*D = total distance in km. = ± 20 mm = 14.6mm @ 0.0146m
true error < permissible error. Levelling work can be accepted.
reduced level of last point – reduced level of last bench mark with known reduced level
Correction
= 0.002
= 0.00014m per FS
Actual reduced level = reduced level±correction
Reduced level Correction Actual reduced level Remarks
2.014 0.000 2.014 BM1=2.014 2.146 0.00014x1= -0.000 2.146-0.000= 2.146 CP1 2.035 0.00014x2= -0.000 2.035-0.000= 2.035 CP2 2.030 0.00014x3= -0.000 2.030-0.000= 2.030 CP3 2.076 0.00014x4= -0.001 2.076-0.001= 2.075 CP4 2.026 0.00014x5= -0.001 2.026-0.001= 2.025 CP5 2.209 0.00014x6= -0.001 2.209-0.001= 2.208 CP6 2.081 0.00014x7= -0.001 2.081-0.001= 2.080 TBM 2.078 0.00014x8= -0.001 2.078-0.001= 2.077 CP7 2.071 0.00014x9= -0.001 2.071-0.001= 2.070 CP8 2.111 0.00014x10= -0.001 2.111-0.001= 2.110 CP9 2.088 0.00014x11= -0.002 2.088-0.002= 2.086 CP10 2.172 0.00014x12= -0.002 2.172-0.002= 2.170 CP11 2.172 0.00014x13= -0.002 2.172-0.002= 2.170 CP12 2.016 0.00014x14= -0.002 2.016-0.002= 2.014 BM1=2.014 Correction = true error
Discussion
Based on analysis, the End closure value which has being calculated is 0.002M. The allowable closure value for this levelling is 0.0146M. In levelling, working data can only be accepted when it end closure value is not more than allowable closure value. Hence, the data for this levelling can be accepted because its End closure value is below the allowable closure value. It is because of an error has occur when transferring the height of the benchmark to another benchmark.
There are few factors which contribute to an error when doing this levelling. One of the factors that can cause an error is the technical problem on the levelling telescope. levelling telescope, is not in parallel position even though the student has adjusting its bubble in centre position. Two-Peg Test is a surveying operation that being carried out to determine if the levelling bubble and telescope line of sight are in parallel condition. If the collimation error still be obtain when doing this Two-Peg Test.
Others factors that contribute to an error when doing this levelling is the parallax error. This parallax error happens due to mistaken that have being done by student when taking the reading on the staff, wrong reading on staff rod has being taken and recorded. To eliminate this parallax error, the focusing screw must be adjusted in order to get a sharp and clear image on the rod staff that being observed. Besides that, the eyepiece of the instrument must be adjusted in order to get a sharp and clear image of the cross hairs.
An error can also come from the rod staff. To overcome an error when taking the reading of levelling, students must ensure that the bottom of sole plate on rod staff does not become encrusted with mud and dirt especially when turning the rod staff to become FB from BS during transfer of elevation. Another factor is the staff is not pick at straight point it is because the factor from wind that contribute the staff position. So, this is the factor why the error is a obtain when the travelling work was we do it.
Conclusion
Leveling is the process to measure the different in height of a point in the earth surface that use a datum as a reference by using level and staff. The aim of this leveling work that we had carried was to find reduced level from known point (benchmark or temporary benchmark) to unknown point. Before we start with leveling, we need to do two peg test to determine if the leveling bubble (bubble axis) and telescope line-of-sight (line of collimation) are parallel and from the process, the collimation error that we gets is 0.001m or 1mm , this is acceptable as the value for the error must be 1mm per 20m. To check the survey accuracy, we had checked the arithmetic of our level reduction and the different of BS and FS is equal to different of RL (last BM) and RL (start BM) that is 0.002, as it is equal hence, the survey accuracy is
acceptable. Our calculated misclosure limit was 14.59mm and misclosure calculated from the different of RL and ARL is 2.0 mm. As the misclosure is smaller and below the
misclosure limit, hance the leveling work is acceptable. The correction for this work was 0.00014m per FS.
REFERENCE
Geomatics Engineering Module 1, Unit UTHM Publishing, 2005 Mr. Anuar bin Mohd Salleh, Lecturer subject Geomatics Engineering 1 Mr. Masiri Bin Kaamin, Admin
Website : - http://en.wikipedia.org/wiki/Levelling
- en.wikipedia.org/wiki/Levelling
- www.thefreedictionary.com/levelling
- en.wiktionary.org/wiki/leveling
- http://www.levelling.uhi.ac.uk/tutorial1_1.html
Book Basics Surveying Engineering, publisher: Abdul Hamid Mohamed Civil Engineering text book form 4.
