You could walk a barometer up to the surface to measure the pressure altitude difference. Many smartphones have integrated barometers; apps such as phyphox [0]

You could walk a barometer up to the surface to measure the pressure altitude difference. Many smartphones have integrated barometers; apps such as phyphox [0] wrap these sensors in a way that's convenient for small physics experiments. [0] https://arxiv.org/abs/1804.06239 https://arxiv.org/abs/1804.06239 ("Advanced tools for smartphone-based experiments: phyphox")