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 ES 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%.
The values are kept in the memory even after power is OFF.
12.1 Calculation of Atmospheric Correction
By inputting the temperature and air pressure values, the atmospheric correction value is calculated
using the following formula and set into the memory. (humidity is regarded as 50%)
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.
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.
•
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
•
If the weather correction is not required, set the ppm value to 0.
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
----------
×
=
