12-1
12 SETTING ATMOSPHERIC CORRECTION
12
SETTING ATMOSPHERIC CORRECTION
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 GM is designed so that the correction factor is 0 ppm at an air pressure of 1013.25hPa, a
temperature of 15
°
C, and a humidity of 50%.
The values are kept in the memory even after power is OFF.
12.1 Calculation of Atmospheric Correction
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 instrument is designed so that the correction factor is 0 ppm at an air pressure of 1013.25 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 instrument 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
0.294280 p
×
1 0.003661 t
×
+
------------------------------------------
–
0.04126 e
×
1 0.003661 t
×
+
------------------------------------------
+
Atmospheric Correction Factor (ppm) =
E
6.11 10
7.5 t
×
(
)
t 237.3
+
(
)
----------------------------
×
=
e
h
E
100
----------
×
=
