Use of scale correction: By entering a scale correction, reductions proportional to distance can be taken into account.
• Atmospheric correction.
• Reduction to mean sea level.
• Projection distortion.
Atmospheric correction D1
The slope distance displayed is correct if the scale correction in ppm, mm/km, which has been entered corresponds to the atmospheric conditions prevailing at the time of the measurement.
The atmospheric correction includes:
• Adjustments for air pressure
• Air temperature
• Relative humidity
For highest precision distance measurements, the atmospheric correction should be determined with an accuracy of 1 ppm. The following parameters must be redetermined:
• Air temperature to 1 °C
• Air pressure to 3 mbar
• Relative humidity to 20 %
Air humidity
The air humidity influences the distance measurement if the climate is extremely hot and damp.
For high precision measurements, the relative humidity must be measured and entered along with the air pressure and the temperature.
Air humidity correction
Index n
Formulas: Formula for visible red laser
D1 Atmospheric correction [ppm]
p Air pressure [mbar]
t Air temperature [°C]
h Relative humidity [%]
x (7.5 * t/(237.3 + t)) + 0.7857
If the basic value of 60 % relative humidity as used by the EDM is retained, the maximum possible error in the calculated atmospheric correction is 2 ppm, 2 mm/km.
Reduction to mean sea level D2
The values for D2 are always negative and are derived from the following formula:
Projection distortion D3
The magnitude of the projection distortion is in accordance with the projection system used in a particular country, for which official tables are generally available. The following formula is valid for cylindrical projections such as that of Gauss-Krüger:
In countries where the scale factor is not unity, this formula cannot be directly applied.
Atmospheric corrections °C
Atmospheric corrections in ppm with temperature [°C], air pressure [mb] and height [m] at 60 % relative humidity.
Atmospheric correction °F
Atmospheric corrections in ppm with temperature [°F], air pressure [inch Hg] and height [ft] at 60 % relative humidity.
