Altitude measurement in Polar Heart Rate Monitors
Altitude and temperature measurement:
The Polar S725/S725X/S7201/S710i/S625X calculates altitude by using the standard average altitude in certain air preasures according to the ISO 2533. The air pressure is measured and temperature compensated every second. The receiver displays altitudes between - 448m - 7590m or between - 1600ft - 24900ft with at least 1m/5ft resolution. The reference point's adjustable altitude range is +- 610m/2000ft from the last measured value.
The altitude readings are based on the measurement of atmospheric pressure, essentially a barometer. Since air pressure changes constantly due to various weather conditions, the calculated altitude of a certain spot may fluctuate with the weather.
Variation of high- and low- pressure can cause a change of up to 80 mbars which means approximately 700m/2000ft change in the calculated altitude. For example in sea level it is normal to have a pressure change of 900-1500 mbars, which can cause +/- 100 meter variation in the measured altitude, which is normal for altitude readings based on air pressure.