The altitude value mainly comes from the barometric altimeter of iGS630.
Since the air pressure will be greatly affected by the climate and temperature, the absolute altitude will also change with the climate and temperature.
iGS630 can be be automatically calibrated when GPS signal is located.
And the altitude value can be manually calibrated.
Meanwhile, the relative altitude measured by the barometric altimeter is highly accurate.
The calculation of the slope, total height and total drop height is accurate, and the reference value of these data for cycling will be greater.