Thanks for the idea! I found a variety of altimeter apps for the phone; I'll have to try just walking around my yard with one (or several), then see if it's possible to get elevation data on a map from such an app.
I'd suggest getting an app like Sensors Multitool for Android which gives you the raw output of your phone's sensors. I tried one of the altimeter apps a few years ago to see how tall a big hill was, and the value that the app reported turned out to be far off the actual value. There are various techniques for mixing and cleaning data from the GPS and barometric pressure sensors and a premade altimeter app (if it's not open source) obfuscates what's really going on with the values you'll actually end up seeing if you create an app for your microfarming project.
Also be aware that different phones have different levels of support for non-GPS location services. I was surprised to learn recently that my Pixel 3 can use data from the US, EU, and Russian satellite geolocation systems for better coverage/precision.
Thanks for the tips! I don't need accurate readings, just precise ones (so the map can be internally consistent; here's what I drew manually for my property: https://i.imgur.com/2ZGmB1M.png). I'll keep in mind what you suggest when I am ready to develop that part of the app.
Well, I tried both Accurate Altimeter and Sensors Multitool, and I must say I'm disappointed. The values varied by 20 meters! In reality, my property varies by 2 to 3 meters at most.
I'll keep thinking about how to solve the problem.
