One way to do this is to add a GPS device to the Heltec, and send GPS data.
When the data gets to the backend, you can compare the GPS co-ords to the signal strength and time and store the data appropriately (perhaps via cloud MQTT / cloud MongoDB?) .
You could also set up “node red” to do this and display signal traces as lines via google maps.
You could put the device in your car and just drive around your area to collect the data.
These links should help
*edit - added from Nic’s post (267)! https://www.thethingsnetwork.org/labs/story/heltec-lorawan-gps-quick-start1
Others
https://ttnmapper.org/faq.php
https://www.thethingsnetwork.org/forum/t/has-anyone-tried-sending-gps-coords-to-ttn/8280/7