Set a profile for when the phone is ringing. The caller’s number will be placed into a variable. Use the task above as a template to send a command to an Item to trigger a rule to do what you want it to do in OH.
Look up presence detection, OwnTracks, IFTTT, or if you want to stick with Tasker, look at AutoLocation.
Tasker is able to read and report the Pressure reading on those phones that have them. Temperature and Humidity as well.
GPS really is not that fast nor accurate for something like that. If your wifi extends out to the garden, you should look into FIND which can do very precise location of devices based on wifi signal strength seen by your phone. If you want to try the GPS route, see comment above about presence detection.