APPLE's GPS ACCURACY / PERFORMANCE for iPhone 7 onwards is considerably worse than for the iPhone 6 ... my story might appear off track for this thread, but I suspect it is related and could well be the real reason for accuracy problems rather than 'compass calibration' per say ... this tale is not a problem with Cachly, but an iPhone issue.
In July 2017 after 'upgrading' from iPhone 6 to iPhone 7 Plus I noticed difficulty in homing in on some caches, trouble was being hundreds of miles from home immediately after changing the phone I couldn't be sure if the area had a 'weak' signal. Over time I was still having problems and also noticed instead of seeing the +/-5M satellite accuracy I was used to with my old iPhone 6, for the 7+ it never went below +/-8M and was very variable. Some months later out with a friend and his iPhone 6, he was laughing at me on & off all day saying I had lost the caching plot these days and why was I looking over there when the cache was here! Sometimes I was up 20/30M out!!
Whilst outdoors and using the Apple or Google Map App, my iPhone 7+ 'blue dot' would wander far & wide maybe 100M up the road & back - the pulsating circle around the blue dot would fluctuate wildly, increasing & decreasing in diameter. In direct comparison, the blue dot for my old iPhone 6 remained very stable at the current location, with only a small diameter pulsating circle indicating good stable satellite accuracy.
Eventfully I contacted Apple and initially experienced the anticipated wall of silence, but after a few calls, by chance, I was through to a more senior team member who took interest. He spoke to Apple's engineering team and had me carry out all manner of tests whilst noting data, later installing tracking software on my phone (I'm very trusting!). Nothing came back as a "real" reason for the change in GPS performance, but eventually they offered to replace the handset despite mine being well out of warranty by then. Initially the replacement phone didn't perform any better, still showing +/-8M satellite accuracy at best, but over time accuracy could settle to +/-6M or very occasionally/briefly to +/-4M. The blue dot/accuracy circle was also a little more stable than for the original 7+ handset, but absolutely nothing like as stable as the trusty iPhone 6! I had a good rapport with the Apple guy and even persuaded him to install a GPS Accuracy App on his own iPhone X ... his X also never showed better than 8M satellite accuracy and the blue dot not very stable!! I gave up at that point and these days take my iPhone 6 and/or Garmin Montana as backup on important caching trips, just in case. I suppose the positive that came out was me still finding caches despite my iPhone7+ rather than with it!! The poorer performing iPhone7+ has perhaps made me a better cacher? Mmmmm, maybe!
Seems to me that Apple changed either the GPS chip, or way they handle the information, after iPhone 6 ... so I have little confidence that any models 7 onwards will ever be as good as the 6 and earlier (my son's iPhone 5 GPS performs same as my iPhone 6). Real shame and crazy why Apple would deliberately dumb down the GPS performance. Hopefully at some point they will rethink their design and go back to the previous good GPS performance.
So, draw your own conclusion whether "compass calibration" or other system is needed, or whether Apple should restore iPhone's to their previously good general GPS performance.