Joined: Aug 2010
Posts: 12,717
Likes: 2,104
From: Madison, WI
Bikes: 1961 Ideor, 1966 Perfekt 3 Speed AB Hub, 1994 Bridgestone MB-6, 2006 Airnimal Joey, 2009 Thorn Sherpa, 2013 Thorn Nomad MkII, 2015 VO Pass Hunter, 2017 Lynskey Backroad, 2017 Raleigh Gran Prix, 1980s Bianchi Mixte on a trainer. Others are now gone.
One of my Garmin GPS units has a pressure sensor, but the unit attempts to self calibrate the elevation by using elevation data from GPS.
I am not familiar with the app you used and my Android phone does not have a pressure sensor. But I am surprised that whomever wrote the software for that app did not try to incorporate a self calibration based on GPS elevation data.
Since your X axis is distance, I am surprised there is no big jump late in the graph when you stopped to rest for several hours, as I would have expected the barometric pressure to continue to change.
I have a couple old Casio watches that are over a couple decades old that have no GPS electronics, thus the pressure sensor in them has nothing to calibrate to. The plot you have is similar to what I would expect from my uncalibrated Casio watches.