Field test mode -- which is accessible by typing *3001#12345#* on your iPhone's keypad -- is a secret door to hundreds of mobile phone measurements.
Received signal reference power (RSRP) and received signal reference quality (RSRQ) can give you a feel for how much signal is available and its usability. And I was curious how the readings might vary as you stroll around the city.
Below is a simple plot of the latitude and longitude survey coordinates, followed by something much prettier to test the concept.
Many more data points are required to tell the full story, but it was a fun to get Jupyter Labs (a great environment for putting Python to work on data) up and running. I enjoyed using Folium too -- it's a powerful library for visualising and interacting with geospatial data.