• 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 
The index n is calculated from the formula of the IAG Resolutions (1999), and is
valid for:
Air pressure p: 1013.25 mbar
Air temperature t: 12 °C
Relative air humidity h: 60%
Formulas: Formula for visible red laser 
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 DD2 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 corrections °F: Atmospheric corrections in ppm with temperature [°F], air pressure [inch Hg] and height [ft] at 60% relative humidity.