SUUNTO X9i - Manual (Page 29)

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3.3.5. Reference altitude
To ensure that the altimeter shows correct readings, you first
need to set an accurate reference altitude. To do this, go to a
location whose altitude you have checked, for example, with
the help of a topographical map, and set the reference altitude
to match the map reading.
NOTE: If you do not know the current altitude, go to the
Position display and check the altitude with the GPS. The
longer the GPS is on, the more accurate the altitude reading
will be.
Setting the reference altitude
To set the reference altitude:
1.In the Set menu, Reference is the first menu item. Press
ENTER to select it. The reference menu opens.
2.Altitude is the first menu item. Press ENTER to select it. The
Altitude display opens. The previously set altitude is
highlighted.
3.Change the altitude with UP/DOWN and press ENTER. The
reference altitude is now set and you return to the Reference
menu.