Suunto Observer - 3.6 Air Temperature Effect on Altitude

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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). Conse-
quently, 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 pres-
sure, the reading of the pressure sensor is always the same independent of the units own tem-
perature. 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
Table 3 shows an example in which the temperature offsets are negative. This time imperial
units are used. The reference altitude is set at 3280 feet. At 9840 feet the altitude difference is
6560 feet and Observer shows 100 feet too much (-14 ºF * 6560 ft * 0.0011/ºF = -100 ft). Your
actual altitude is thus 9740 ft.