I am using openstreetmap. In my application, I use GPS information to locate the user. In cases of bridges and overpasses, latitude and longitude are not enough, since they are the same under and on the bridge. I thought maybe to use GPS altitude along with elevation data, is it possible? What is the elevation in these cases? is there elevation data for all geographical layers in the same GPS location?
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Unfortunately GPS is much more imprecise on the vertical axis as it is horizontally. It can easily differ by twenty meters, which makes it rather unusable for telling whether you're above or below for most bridges.
Some GPS devices also have a barometric sensor which can give more precise results, but these have to be calibrated on a regular basis.
And last not least: most OSM data does not have actual elevation information (partially because of the technical reasons described above), but rather just "this road is one layer above that road", so even when knowing your actual true elevation you could still not tell what logical level you're on ...