Many location-based applications allow the use of an external GPS receiver connected via Blue Tooth or WiFi (in some rare cases even a RS-232 Serial Port). Bit surprised that iNat doesn’t include that functionality. As the built in GPS in iOS and Android devices performs quite poorly when not in cell range, which is where location accuracy is most important, the ability to configure the app to accept location from an external receiver is critical. As most consumer-grade GPS allow connection via Bluetooth to stream the NMEA messages, this is not a fringe use-case. Would also be handy to accept NTRIP streams as well, for users in possession of such equipment (rare use-case).
I’m always curious when people ask about this stuff. All the iPhones I’ve had since the 4 regularly get 5 to 10 m accuracy outside of cell service and when I compare with Garmin gps or google base maps the iPhone observations, at least, are really solid. Android (Samsung 5) is a bit worse, maybe 10m but takes longer to lock in too. What precision are you looking for?
One suggestion: If you put your cell phone in Airplane Mode and use only the built in GPS for your location service, you should get results just as good as any external receiver. Some folks also install a 3rd-party GPS app and keep it running in the background (while in Airplane Mode) to ensure the built in GPS stays active. Also, turn off Auto-sync in the app. Then later when you have a good cell or WiFi connection, take it out of Airplane Mode and upload the accumulated observations.
Maybe they’re referring to submeter or subfoot receivers?
I have been using my Samsung S9+ in the field, totally out of range of any cell towers, for the last 18 months or so and have had no issues with coordinates being easily within 5 to 10 meters accuracy. I also find that using a GPS tracker at the same time does improve the accuracy when in dense forests or on overcast days.