Altimeter

"My backpack was already packed, and I'd already gotten the other supplies together, like the altimeter and granola bars and swiss army knife I'd dug up in Central Park, so there was nothing else to do" (315).
a sensitive aneroid barometer that is graduated and calibrated, used chiefly in aircraft for finding distance above sea level, terrain, or someother reference point by a comparison of air pressures. (dictionary.com) 

The planes altimeter let the pilot know that he was flying at 6,500 feet in the air