## Two Peg TestAll instruments are subject to errors. The checking of the instrument (level) is therefore important. The main error is where the line of sight is not parallel to the horizontal line of collimation. In this case your levels will not be correct. A test for checking the level is known as the two peg test. This test determines the amount of errror and if an error occurs notify the technician (the level must be serviced).
- Establish 2 points approximately 50 metres apart on level ground as
shown below. Set the level half way between the 2 points.
Figure 2 - Move the level as close as possible to one of the peg. (in the case
above 'Peg A').
Take the 2 staff readings again. - If the difference in height is the same the level is okay. If not,
- Take the 2 staff readings. In our example an error will exists (line of sight does not coincide with line of collimation).
[ top of page ] Gid leveling is used for site investigation, for drawing contour line and for the easy calculation of volumes. The opposite figure shows a typical survey of a site using grid levels.
The area of the site is divided into a number of squares 5 times 5
metres (triangels or rectangles can also be used) and levels are taken
at corner points. The grid levels enable us to calculate the volume of
material above or below a certain reduced level (RL) and to draw contour
lines. [ top of page ]
Figure 4 below shows the site in isometric view. As can be seen the horizontal plane at RL = 12.900 lies between the RL = 10.000 and RL = 15.831. If a house is placed on this level then some cutting and some filling would be required. However, if the FGL (finished ground level) is at RL ≤ 10.00 then all soil of the site need to be removed and if RL ≧ 15.831 soil must be transported to the site. Figure 4 The two grid elements [ top of page ] The work of getting the average of the spot levels can be simplified by the use of a suitable table as illustrated below: >
Step 2 Find the difference between the mean height (MH) and the proposed finish ground level (FGL). a) the difference can be positive (+ve) or negative (-ve). b) if the mean height is greater (+ve) than the finish ground level (FGL) then it is a cut. c) if the mean height is smaller (-ve) than the finish ground level (FGL) then it is a fill . MH
< FGL = FILL
In the isometric view the FGL = 12.900 [ top of page ] Contour lines
Figure 5 There are two ways to plot contour lines: a) by estimation b) by calculation a) is the quickest method of plotting the contours. Estimate by visual inspection the position of a contour between two adjacent spot levels. b) is more accurate. Similar triangle rule is used to calculate the contour line. This principle is show in Figure 6 below and is generally used for all your exercises and Assignments. Using similar triangles: Using the above equation the actual calculation for the contour lines 11.000 m and 11.500 m is:
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Assignment (Volumes and contour lines)
The opposite figure shows a building site. The lot is 50 × 40 metres and has spot levels at 10 metre intervals as indicated.
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