1 II. INTRODUCTION Leveling in which a continuous check is maintained on the accuracy of the leveling procedure. It is called double rodding because it can be done most conveniently by two rodmen. However, it is possible to carry out the procedure using only one rodman. In double rodding, you determine the HI at each setup point by backlights taken on two different TPs. If no mistake or large error has been made, the result will be two HIs that differ slightly from each other. Elevations computed this way will also differ slightly. In each case, the average is taken as the elevation.
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II. INTRODUCTION
Leveling in which a continuous check is maintained on the accuracy of the leveling
procedure. It is called double rodding because it can be done most conveniently by two
rodmen. However, it is possible to carry out the procedure using only one rodman. In
double rodding, you determine the HI at each setup point by backlights taken on two
different TPs. If no mistake or large error has been made, the result will be two HIs that differ
slightly from each other. Elevations computed this way will also differ slightly. In each case,
the average is taken as the elevation.
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III. OBJECTIVES AND INSTRUMENTS
OBJECTIVES
1. To develop skills in determining the difference in elevation between two points
without the need of returning the circuit to the original point of reference by using
two observation points every time BS and FS are taken.
2. To master the art of using the precise level during fieldwork.
3. To acquire the confidence of working with one’s party or group and to be fully
responsible in the performance of the assigned task.
INSTRUMENTS
1. Precise level 2. Chalk
- An instrument similar to an ordinary - A soft compact calcite with
surveyor's level but capable of finer varying amounts of silica, quartz,
readings and including a prism arrangement feldspar, or other mineral impurities,
that permits simultaneous observation generally gray-white or yellow-white
of the rod and the leveling bubble. and derived chiefly from fossil
3. Leveling Rod 4. Plumb bobs
- used with a levelling instrument to - is a weight, usually with a pointed tip
determine the difference in height on the bottom, which is suspended
between points or heights of points above from a string and used as a vertical
a datum surface. reference line
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IV. PROCEDURE & COMPUTATIONS
A. Determination of the difference in elevation between two points, which are at a
considerable distance from each other
1. The professor designates the locations of two benchmarks BM-1 and BM-2, which is
about 300m to 500m away from each other and of considerable difference in elevation.
2. Setup the level on a suitable location and take the BS of BM-1 (point of known elevation)
while the rod man is holding the level exactly at BM-1. Compute the height of the
instrument by using the formula: HI = Elevation of the point + BS.
3. The rod man now transfers to another location, preferably the same distance of the BM-1
to the instrument as the distance of the first turning points (namely TP1L and TP1H) to the
instrument.
4. The rod man must mark two points on this location and call them TP1L and TP1H with a
chalk or marking pins. Preferably, these two points must be of considerable difference in
elevation but quite near each other.
5. The instrument must take two FS’s one on each TP1L and TP1H and record the readings
on the computation sheet provided for in this fieldwork.
6. Compute the elevations of these TP1L and TP1H using the formula: Elevation of TP1L =
HI – FS of TP1L and Elevation of TP1H = HI – FS of TP1H. Record the results in the
computation sheet.
7. The instrument man must now transfer the level pass of the two TP1 and along the route
to BM-2.
8. After leveling the instrument firmly, two BS’s must be taken one on each TP1L and
TP2H. Record these readings.
9. Compute the new height of the instrument on its current location by basing it from the
two turning points. Use these formulas: HI = Elevation of the TP1L + BS of TP1L and HI
= Elevation of the TP1H + BS of TP1H. These two computed values of HI must be very
near each other.
10. Again the rod man transfers to another location, along the same route preferably the same
distance of the instrument from TP1’s, and should now call them TP2’s.
11. The instrument man gets two FS’s one on each TP2L and TP2H and record these rod
readings on the computation sheet.
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12. Compute the elevations of these TP2L and TP2H using the same previous formulas.
13. These steps must be followed closely until the leveling work will reach BM-2. Two
foresights will be recorded for BM-2 and two independent elevations of BM-2 will be
computed. However, these elevations of BM-2 must be very near each other. The mean
of the two elevations will be considered the most probable elevation of BM-2.
COMPUTATION:
The computation of sample field notes is done in accordance with the steps listed hereunder:
A. Computation of the height of the instrument
The height of the instrument is always equal to the elevation of the point where the back
sight is being observed plus the elevation of the point itself.
B. Computation of the height of the instrument
The elevation of the turning point is always equal to the height of the instrument minus the
foresight of the TP being observed.
C. Computation of the most probable elevation of BM2
The most probable elevation of BM-2 is the mean elevation of BM-2 between the computed
values from the last turning points low and high.
Most Probable Elevation of BM – 2 = Mean Elev. Of BM – 2