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Now 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 Wristop Computer 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 Wristop Computer shows 80 m too little (20 ºC
* 2000 m * 0.002/ºC = 80 m). Your actual altitude is thus 3080 m.
Set reference altitude (real altitude)
Displayed altitude
Real outside temperature
Normal (table) temperature
Temperature offset (= real - normal)
Sum of temperature offsets
Table 2. Example using meters and Celsius
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 Wristop Computer shows
100 feet too much (-14 ºF * 6560 ft * 0.0011/ºF = -100 ft). Your actual altitude is thus 9740 ft.
Lower point Higher point
1000 m
3000 m
+17.5 ºC +6.5 ºC
+8.5 ºC -4.5 ºC
+9 ºC +11 ºC
+9 ºC + +11 ºC = 20 ºC
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