Once the crops have grown taller than the flags you originally installed in the field plots? How to do you find your plots within a farm when you return to collect more data?
Do you mean a GPS waypoint application? I'm sure you can find numerous examples in the app stores. Alternatively, a handheld GPS might be old-school these days but totally* reliable and doesn't need a mobile/cell tower connection. Precise enough to re-locate the centre of a field plot.
*"totally" depending on weather, battery and reliability of originally entered waypoints. (I've never had much success with phone-based GPS).
I would also look for a portable GPS. We tested both the phone app and the GPS during an on-farm soil survey. Working outside for long hours in cold conditions and away from a power source was a problem for my team and I when using cell phones. You certainly don't want to run out of battery in the middle of an unharvested cornfield (true story)!
I believe this GPS Tracks (https://apps.apple.com/us/app/gps-tracks/id425589565) is an updated version of an App I used before (MotionX GPS) for something similar :)