Liz is correct. Most smartphones have very good GPS hardware. If you take a photo at a sampling location, that photo should have location coordinates associated with it (although that function can be disabled). Many apps allow GPS waypoints to be saved anytime and continual tracks to be saved (GPS data saved every few seconds). My cheap Moto phone and a free app allow pretty accurate location data to be saved far from cell service even when the phone is in a pocket (research note). Some apps allow a waypoint to include information gathered over a minute or more and then averages the location data to make a more reliable waypoint.
have you heard of the maps.me app? it's great because it works offline as well.
Is this a question? Click here to post it to the Questions page.
I have not tried MAPS.ME. It looks like it allows you to save base map data to your phone while you have a data connection, and then display the maps after the data connection is lost. I think this is a common feature of GPS apps. A less common feature of MAPS.ME is that it is open source.