So, part of this is an experiment. I don't think anybody's done such fast and distributed crowdsourcing of weather data before, so I'm not sure how useful the data will end up being. As far as a single user entering a building, taking elevators, etc, I'm not concerned since those data points will stand out from the nearby readings and can easily be discarded. However, a point raised in another comment here is, what if most users in an area (say Manhattan) are constantly changing elevation. That very well may pose a problem. We'll see. :)