2.9.5
DEFINITIONS & FORMULAS CHAPTER 9 GEODIMETER SYSTEM 600
As the speed of light varies slightly when passing through
different air pressures and temperatures, an atmospheric
correction factor must be applied in order to achieve the
correct distance. This atmospheric correction factor is
calculated according to the following formula:
Geodimeter System 600 calculates and corrects for this
automatically. Please ensure that the instrument is working
with the correct units, MNU 65, Unit.
Examples:
To show the significance of the different units used for
calculating the ppm factor let's take a look at the following:
Atmospheric Correction
p
(273.15 + t)
ppm = 274.41 – 79.39 x +11.27 x
p = pressure in millibars
p
w
= partial pressure of watervapour in millibars
t = dry air temperature in degrees centigrade (Celsius)
The partial pressure of watervapour (p
w
) is calculated
according to the following:
p
w
(273.15 + t)
17.269 x t
237.3 + t
p
w
= x 6.1078 x e
OR
h
100
( )
17.269 x t'
237.3 + t'
p
w
= 6.1078 x e – 0.000662 x p(t – t')
( )
p = pressure in millibars
p
w
= partial pressure of watervapour in millibars
t = dry air temperature in degrees centigrade (Celsius)
t' = wet temperature in degrees centigrade (Celsius)
h = relative humidity in %
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