What
is Geodetic Leveling ? ![Bar coded calibrated rod](Images/leveling2.jpg)
Vertical surveying is the process of determining elevations
above mean sea-level. In geodetic surveys executed primarily for mapping,
geodetic positions are referred to an ellipsoid, and the elevations of
the positions are referred to the geoid. However, for satellite geodesy
the geoidal heights must be considered to establish the correct height
above the geoid. Precise geodetic leveling is used to establish a basic
network of vertical control points. From these, the height of other positions
in the survey can be determined by supplementary methods. The mean sea-level
surface used as a reference (vertical
datum) is determined
by averaging the hourly water heights for a specified period of time at
specified tide gauges.
There are three
leveling techniques:
differential,
trigonometric,
and barometric. Differential
leveling
is the most accurate of the three methods. With the instrument locked
in position, readings are made on two
calibrated staffs held in an upright position ahead of and behind
the instrument. The difference between readings is the difference in elevation
between the points.Trigonometric
leveling involves measuring a vertical angle from a known distance
with a theodolite and computing the elevation
of the point. With this method, vertical measurements can be made at the
same time horizontal angles are measured for triangulation. It is, therefore,
a somewhat more economical method but less accurate than differential
leveling. It is often the only practical method of establishing accurate
elevation control in mountainous areas. In barometric leveling, differences
in height are determined by measuring the differences in atmospheric pressure
at various elevations. Air pressure is measured by mercurial
or aneroid barometer, or a boiling point
thermometer. Although the accuracy of this method is not as great
as either of the other two, it obtains relative heights very rapidly at
points which are fairly far apart. It is used in reconnaissance and exploratory
surveys where more accurate measurements will be made later or where a
high degree of accuracy is not required.
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