Barometric, pressure altitude is used in aviation, because airplanes use altimeters which are based on barometric pressure. As others have said, the pressure readings will vary based on the current local pressure. In a plane, you generally set the altimeter to the KNOWN level of the airport before you take off, and/or to the known current pressure at the airport (resulting in the same result) and then, as you fly, you update your altimeter setting every 30 minutes or so, based on the barometric pressure at the closest airport Look up “AWOS” (automated weather observation system) or “ASOS” (Automated Surface observation system) for an airport near you, which you can phone into, or listen on a scanner, and you’ll hear “Current altimeter is 29.92” or something, which tells the pilot what to set.
To do what you’re trying to do, you really care about the DIFFERENCE between the pressure at the base of the building, and at the top, or elevator location. To do this, you can permanently install a sensor at a known level (likely top or bottom for convenience) and then use a radio or wired communication to read the pressure at the known location and the pressure at the movable location at the same time, and subtract to get the difference. You can calculate the height difference of the two sensors fairly accurately with the difference between the two sensors. If you have an airport with AWOS reasonably close by, and can use the internet to pull the pressure from that location as your “known” pressure, you may be able to use that, but just know that the error will be increased when there’s a pressure front coming through, because the pressure will go up or down at one location before the other.
If interested, you can use https://www.airnav.com to find the AWOS or ASOS of a local airport… For example, https://www.airnav.com/airport/KBWI shows that ASOS is available at PHONE 410-691-1278. Anyone can call that to listen to the current weather.
Hope that helps!!