SURFACE LEVELING, AREAS

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SURFACE LEVELING, AREAS VOLUME DETERMINATION
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Fig. Surface Leveling
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PRECISE LEVELING To achieve higher order of
accuracy, special care must be performed to
minimise errors in leveling. For the most
accurate work, the so called precise leveling
method is used. The procedure and the equipment
used in precise leveling are more refined and
details are paid more attention. Special levels
and rods are used. The rods have scales
graduated on invar strips, which are only
slightly affected by temperature variations.
Precise level rods (also call invar rods) are
equipped with rod bubbles to facilitate plumbing,
and special braces aid in holding the rod steady.
Rods usually have two seperate graduated scales.
This helps us to check the readings and blunders.
As in the following figure, the rod has two
centimeter graduations on the Invar strip, with
the right one precisely offset from the left by a
constant.
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By using two double scaled rods, it is possible
to perform a single run, double-simultaneous
leveling operation with the minimization of
systematic errors as well as detection of
blunders.
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Cloudy weather is preferable for precise
leveling, but an umbrella can be used on sunny
days to shade the instrument and prevent uneven
heating which causes the bubble to run. Precise
work should not be attempted on windy days. For
the best results, short equal BS and FS distances
up to a maximum of 50 m are recommended and their
lengths balanced to within 2 m at each setup.
Precise leveling requires good-quality turning
points. This requirement leads the use of ground
plates in the field. Line of sight should not
pass closer than about 0.5 m from the ground to
avoid refraction. Reading at any setup must be
completed in rapid succession otherwise changes
in atmospheric conditions might significantly
alter refraction characteristics between them.
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Circuit closures are calculated in order to get
blunders and errors that larger that the boundary
values. After all mistakes and systematic errors
are removed from the measurements then a
least-squares adjustment for the entire network
is performed to provide elevations of all
benchmarks. In the modern applications
observations are recorded into a portable
computer or data collector. If the required
programs are in the computer then it is possible
to check immediately any closure or reading
errors occurred during the field operation before
the level is moved to next setup. The recording
the observations makes also easier and faster the
computations and checks in the office.
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PRECISE LEVELING PROCEDURE After checking the
rods are plumb and the BS and FS distances are
equal then go with the following steps on every
level setup 1. Sight the telescope on the
backsight rod and read the center cross hair on
the main graduation. After recording the center
reading then read and record the lower and upper
stadia hairs. Calculate the BS distance. 2. Point
toward the foresight rod and read the center
cross hair on the main graduation. After
recording the center reading then read and record
the lower and upper stadia hairs. Calculate the
FS distance and check the difference from the BS
distance. Also calculate the height difference
obtained from main graduation readings.
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3. While the telescope still focused on the FS
rod, read and record the center cross hair on the
auxiliary graduation. Check the constant between
two graduations of the rod. If the value is
larger then a pre-determined values (e.g. 0.010)
readings should be repeated. 4. Sight the
telescope on the BS rod , read and record the
center cross hair on the auxiliary graduation.
Calculate the height difference obtained from the
auxiliary graduation and check with the one
obtained from the main graduation. Check the BS
and FS distances to balance them on the next
setups.
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Precise level with micrometer (Wild N3)
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Precise level with micrometer (Wild Koni 007)
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Automatic level with micrometer (Zeiss Ni1)