This is from Strava's site

How Altitude data is collected:
Barometric altimeters determine altitude by measuring atmospheric pressure. Measurements can be affected by changes in the weather or the sensor holes getting blocked by water or other debris. Device manufacturers may provide basic calibration procedures. For example, Garmin devices with barometric altimeters allow a certain number of manual elevation points to be set; starting an activity near an elevation point causes the device to use the known elevation as the starting point. Consult the manual for your device to learn about calibration procedures.


GPS altitude measurements are derived from GPS signals and can have relatively high vertical error. They also depend on how many satellites are being used and where the satellites are located in the sky.


We generally consider the data from barometric altimeters to be of higher quality than the data derived from GPS signals and prefer that when processing activities. For devices without barometric altimeters, we consult elevation databases to determine the elevation at each point in the activity. The resolution of these databases can vary based on location. For example, the USGS National Elevation Dataset (NED), which we consult for activities located in the US, generally has a 10 meter resolution though some small areas have a 3 meter resolution. For activities located outside of the US, we consult the ASTER and SRTM databases which have a resolution of 30 meters and 90 meters respectively.


We discard outliers and smooth the elevation data when calculating gain to reduce noise. The amount of smoothing depends on the source of the data - data from barometric altimeters is smoothed less while data looked up from elevation databases is smoothed more. When computing gain, we use a threshold to determine whether or not to include a change in elevation; in this way, small variations in elevation change are ignored while larger changes are included.