Modern phone barometers are MEMS pressure sensors reporting absolute atmospheric pressure in hectopascals with stationary noise on the order of plus or minus zero point three to one pascal root-mean-square. That noise floor sounds tiny until you realize what it represents physically: at sea level, air pressure decreases by roughly twelve pascals per meter of altitude gain, a relationship derived from hydrostatic equilibrium where the pressure change equals density times gravitational acceleration times height change. Twelve pascals of signal therefore corresponds to about one meter of vertical resolution in ideal conditions — good enough to distinguish basement level B2 from B3 in a typical three point two meter per floor garage, but nowhere near precise enough to replace horizontal GPS positioning.

Wheria uses barometer data as a floor hint, not a navigation solution. When you tap Park, the application records pressure P-zero as a session baseline, optionally flagged if you are still outdoors at street level where weather-driven drift has not yet accumulated. On each subsequent barometer sample, arriving at one to ten hertz depending on operating system scheduling, the algorithm computes delta-P equals current pressure minus P-zero, then applies a low-pass filter around zero point zero five hertz to suppress step noise from HVAC fans and walking-induced pressure fluctuations, and a high-pass filter around zero point zero zero one hertz to remove weather fronts that change ambient pressure by two hundred to four hundred pascals per day — enough to falsely suggest you descended an entire floor when a cold front arrived instead.

Where the naive formula breaks

Elevators introduce rapid pressure transients from pressurization and ventilation systems that exceed the rate of change produced by walking down a ramp, causing false floor detections unless the inertial measurement unit reports elevator-like motion and down-weights barometer trust during that interval. Parking with the engine or climate control running shifts local pressure slightly inside the vehicle cabin before you exit. iPhone and Pixel barometers exhibit different absolute offsets — we store per-session baselines at park time rather than attempting aviation-style QNH altimeter correction against a weather service. Pressurized elevator shafts in high-rise towers like Landmark 81 behave differently from open spiral ramps in suburban malls, which is why ktuyen segments barometer regression tests by garage architecture rather than assuming one calibration fits all.

What we display and what we refuse to claim

When delta-P maps to a floor index using user-configured slab height — default three point two meters per level, or values learned from prior visits to the same structure — the interface shows something like "Likely B3" with a plus-or-minus one floor uncertainty band. We never auto-overwrite a floor label the user typed manually, because barometer inference is probabilistic and a wrong automatic label erodes trust faster than no label at all. In Landmark-style open slab garages with three point two meter floor height, median classification error is zero floors when the user walks stairs; elevator-only transitions mislabel roughly thirty percent of the time without IMU activity context distinguishing "walking ramp" from "standing in moving box."

Understanding barometer behavior in garages also explains why we fuse floor hints with inertial activity rather than treating pressure alone as ground truth. A user who rides an elevator from B1 to B3 produces a pressure profile indistinguishable from someone who walked two flights of stairs in ten seconds — unless the accelerometer reports the characteristic vibration and near-zero horizontal displacement of elevator travel. That fusion logic lives in the same pipeline as step odometry and compass heading, and it is why ktuyen logs both "stairs only" and "elevator only" test cases separately in her regression spreadsheet.

You can experiment with pressure-to-floor mapping on our Lab page garage scroll demo, which visualizes the hydrostatic relationship in real time. The barometer feeds into the broader indoor pipeline described in indoor navigation when GPS dies, and its measurements enter the Extended Kalman Filter framework alongside step odometry and compass heading as discussed in Kalman filters for parking. For the sensor fundamentals behind the readings, Phyzix exposes raw barometer data with SI units in its environment instruments section.