As a mountainbiker I use altitude information intensely. Accurate information (current value, total uphill!) is essential. The most accurate altitude information is provided by an air pressure sensor, which is available in my Xperia Z3 compact (and other smartphones).
Currently the calibration of the actual altitude can be done in 2 ways (actually 3, but the following 2 are the most relevant):
1. Automatically (Altitude-Manager --> Air Pressure --> Automatic) by using GPS or(?) SRTM data.
2. Manually (Altitude-Manager --> Air Pressure --> Altitude) by defining an altitude an calibrating the sensor.
The big disadvantage when using #1 is, that imprecise GPS-data (SRTM had no influence in my tests) is used as a basis which has the effect that in case of recording a track the starting altitude is e.g. 200m instead of 400m, rising constantly as GPS-signal gets better and showing >150m uphill and more after seconds/minutes. This makes this setting useless for track recording.
#2 offers perfect values as soon as you have calibrated the air pressure in advance. Disadvantage here is that you have to do it manually with several clicks each time you activate gps or start a track recording (air pressure changes over time and todays values can be quite different from yesterdays...). This prevents also the usage of Locus widget as a quick start for track recording...
My suggestion for proper usage of air-pressure sensors: Let altitude calibration (option #2) be done automatically at GPS-activation when air pressure sensor is used (Altitude Manager --> Settings --> Air Pressure Sensor =On) and Altitude-Calibration is chosen (Altitude-Manager --> Air Pressure --> Altitude = xxx m)!
In my experience >90% of track recordings are for training purposes starting at home at a well known altitude. Or at other places where starting-altitude is known. So Option #2 would be perfect for calibration at GPS-activation resulting in correct altitude values in the statistics of the track.
If user really doesn't know the altitude - well, he can choose option #1...