Non-altimeter GPS units like this use your location to determine your altitude based on the internal map. How can you prove this? Try climbing a tree and watch your altitude NOT change. You might get a bit better results if you load a set of topo maps (instead of base or road maps), but when you are in steep terrain, near cliffs, on bridges, etc. If you really need an accurate altitude, you can either buy a unit with an altimeter or buy a separate device. Personally, I don’t need one.