Sokkia 30R Series, Operator'S Manual, Page: 175 / 178

167
30. EXPLANATION
• Need for atmospheric correction
The SET measures the distance with a beam of light, but the velocity of this light
varies according to the index of refraction of light in the atmosphere. This index
of refraction varies according to the temperature and pressure. Near normal
temperature and pressure conditions:
With constant pressure, a temperature change of 1°: an index change of 1 ppm.
With constant temperature, a pressure change of 3.6 hPa: an index change of
1 ppm.
To perform high accuracy measurements, it is necessary to find the
atmospheric correction factor from even more accurate temperature and
pressure measurements and perform an atmospheric correction.
Sokkia recommends that extremely precise instruments be used to monitor the
air temperature and pressure.
• Finding the average temperature and pressure between two points in different
atmospheric conditions
To precisely determine the atmospheric correction factor, the average
temperature and air pressure along the measurement beam route must be
taken.
Determine the temperature and pressure as follows.
Flat terrain
:Use the temperature and pressure at
the midpoint of the line.
Mountainous terrain :Use the temperature and pressure at
the intermediate point (C).
If it is not possible to measure the temperature and pressure at the midpoint,
take the temperature and pressure at the instrument station (A) and the target
station (B), then calculate the average value.
Average air temperature :(t1 + t2) /2
Average air pressure :(p1 + p2)/2
30.2 Atomospheric Correction for High Precision
Distance Measurement
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