SUUNTO Observer - Manual (Page 31)

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The altitude measurement error caused by an abnormal temperature gradient can be
approximated as follows.
If the sum of the temperature offsets from the normal
temperatures determined at two different altitudes is 1 ºC, the altitude
difference calculated by the Observer is 0.2% off the real altitude
difference (When using imperial units the offset factor is 0.11% / 1 ºF). This is because
the real temperatures are not always the same as the normal temperatures. A higher
than normal temperature causes the calculated altitude difference to be smaller than
the real altitude difference (your mountain ascent was actually higher). Consequently, a
lower than normal temperature causes the calculated altitude difference to be larger
than the real altitude difference (you did not ascend quite as high as displayed).
Table 2 shows an example in which the temperature offsets are positive. In this example,
the reference altitude is set at 1000 m. At 3000 m the altitude difference is 2000 m and
Observer shows 80 m too little (20 ºC * 2000 m * 0.002/ºC = 80 m). Your actual altitude
is thus 3080 m.
All Suunto products with altimeter are temperature calibrated. This means that at
constant pressure, the reading of the pressure sensor is always the same independent of
the unit’s own temperature. However, the air temperature can still effect the altitude
readings as described above.
Table 2. Example using meters and Celsius
Lower point Higher point
Set reference altitude (real altitude) 1000 m
Displayed altitude 3000 m
Real outside temperature +17.5 ºC +6.5 ºC
Normal (table) temperature +8.5 ºC -4.5 ºC
Temperature offset (= real - normal) +9 ºC +11 ºC
Sum of temperature offsets +9 ºC + +11 ºC = 20 ºC