NSA/SONDE/C1 - Incorrect surface altitude
The value of the station altitude (relative to mean sea level) is incorrect in all NSA radiosonde files prior to 2/14/2004. The correct value is 8 m. The incorrect value can be different. Prior to 4/24/2002 (in platform nsaisssonde...) it was given as 12 m. From 4/24/2002 to 2/13/2004 (in platform nsasondewnpn...) it is almost always given as 27 m (sometimes [rarely] 17 m).
Because the altitude computation is based on the pressure measurement the error is linear. A general method to correct the problem is outlined in the following pseudocode: SET SurfAlt = 8. GET Array_of_alt_values SUBSTRACT first alt_value from all alt_values ADD SurfAlt to alt_values