20. CHANGING THE SETTINGS
112
Atmospheric correction factor
The velocity of the light beam used for measurement varies according atmospheric conditions
such as temperature and air pressure. Set the atmospheric correction factor when you wish to
take this influence into account when measuring.
• The OS is designed so that the correction factor is 0 ppm at an air pressure of 1013 hPa, a
temperature of 15°C, and a humidity of 50%.
• By inputting the temperature, air pressure values and humidity, the atmospheric correction value
is calculated using the following formula and set into the memory.
t : Air temperature (
°
C)
p : Pressure (hPa)
e : Water vapor pressure (hPa)
h : Relative humidity (%)
E : Saturated water vapor pressure
• e (water vapor pressure) can be calculated using the following formula.
• The OS 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°C: 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.
It is recommended that extremely precise instruments be used to monitor the air temperature and
pressure.
• Enter the average temperature, air pressure and humidity along the measurement beam route
in "Temperature", "Pressure" and "Humidity".
Flat terrain:Use the temperature, pressure and humidity at the midpoint of the line.
Mountainous terrain:Use the temperature, pressure and humidity at the intermediate point (C).
If it is not possible to measure the temperature, pressure and humidity at the midpoint, take
such measurements 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
Average humidity
: (h1 + h2)/2
282.324
0.294362 p
×
1 0.003661 t
×
+
------------------------------------------
–
0.04127 e
×
1 0.003661 t
×
+
------------------------------------------
+
Atmospheric Correction Factor (ppm) =
E
6.11 10
7.5 t
×
(
)
t 237.3
+
(
)
----------------------------
×
=
e
h
E
100
----------
×
=
