Here is a video and some insights on my experience in working with pressure and depth measurements on Garmin devices. My diving apps are available at www.diveiq.com
https://www.youtube.com/watch?v=rt-yz88q4LE&t=41s
This video shows depth being computed using three different methods for obtaining pressure on Garmin devices.
1) Raw ambient pressure: As measured by the pressure sensor.
2) Ambient pressure: A smoothed, two-stage filtered pressure provided by Garmin which reduces noise and instantaneous variation.
3) Altitude pressure: Pressure derived from the altitude .
Many Garmin watches have a pressure sensor that Garmin uses internally to compute altitude. This sensor's data was only available internally to Garmin software and not made available to third party developers. Third party apps such as DiveIQ's Dive Computer and Freedive apps were forced to convert altitude back to pressure. This reverse conversion works pretty accurately but introduces some lag (delay) because it takes a few seconds for pressure changes to reflect on the altitude. Applications deriving pressure from altitude also need to be capable of coping with altitude decalibration because the altitude measurement provided by Garmin does not solely depend on pressure. The altitude also depends on multiple user settings that Garmin uses to determine if changes in pressure are due to changes in weather or changes in altitude. GPS measurements and manual calibrations are also taken into account. Garmin now provides two pressure measurements. A "raw ambient pressure" value taken directly from the sensor and an "ambient pressure" value smoothed by a two-stage filter that reduces noise and instantaneous variation. This video shows shows the response time and accuracy when using each pressure measurement method to compute depth in meters.