G-Shock GW-9200BPJ-1 - Manual (Page 37)

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• Do not rely upon this watch for altitude readings or perform button
operations while sky diving, hang gliding, or paragliding, while riding a
gyrocopter, glider, or any other aircraft, or while engaging in any other
activity where there is the chance of sudden altitude changes.
• Do not use this watch for taking altitude readings in applications that
demand professional or industrial level precision.
• Remember that the air inside of a commercial aircraft is pressurized.
Because of this, the readings produced by this watch will not match the
altitude readings announced or indicated the flight crew.
How the Altimeter Measures Altitude
The altimeter can measure altitude based on its own preset values, or a
reference altitude specified by you.
When you measure altitude based on preset values
Data produced by the watch’s barometric pressure sensor is converted to
approximate altitude based on ISA (International Standard Atmosphere)
conversion values stored in watch memory.