It is a great description.
But I am not quite understand the ending of your last paragraph, you mentioned that you have successfully landed at the spots which you suspected there might be a interference, dose it mean you still can launch even you were knowing there might be a risk after the successful re-calibration?
Example #1: MAP LINK
Tai Mei Tuk, Hong Kong
The area I wanted to fly was over water, away from any hazards that I could see. The area I wanted to launch from is reclaimed land...concrete, with iron re-bar. No grass or dirt anywhere. I was successfully able to calibrate my drone...but every time I put it on the ground...it would show "compass" interference. Every time I picked the drown up, the interference would disappear and the drone would announce it was ready to fly.
I eventually hand-launched my drone, and flew it with no problem. I also landed it on the ground with no problem...once landed, the drone again showed "interference" and I would not have been able to launch again without hand-launching.
Later I moved to the dam, south of the location marked on the map. Again...the drone showed "compass interference" when trying to launch from the dam, probably due to the amount of iron re-bar in the dam. Again..I hand launched..flew and landed without issue.
There was very little risk flying in this location. I had wide open skies and unobstructed views. The main interference seemed to come from the ground. My assessment was that the risk of experiencing drone failure due to interference by flying here was low.
Example #2: MAP
Unesco World Heritage Site, Hong Kong
It is hard to see elevation on this map... but basically...I had no trouble calibrating and launching from the higher dam, to the northwest of the marker on the map. When I tried to launch from the lower dam (more of a tidal break made of giant concrete and re bar pillars) the drone would not allow me to fly due to "interference" every time I set it on the ground. I eventually hand launched the drone and successfully flew and landed it without incident.
The risk flying in this location (as I headed out over the ocean) is that the concrete pillars and iron could have caused signal interference and I risked losing my drone. I still had line of site as long as I flew my drone high enough... and a fairly good view of the sky for GPS. The dam behind me was very high and could possibly have caused interference as well. My assessment was that the risk of experiencing drone failure due to interference by flying here was moderate.
Example #3: MAP
Upper Mesa Falls in Idaho, USA
I had no trouble calibrating my drone, but when attempting to launch it from the walkway (on the right of the map if you are in Satellite view) I kept getting "compass interference". I eventually found a boulder near by, and was able to launch from there and then land on the walkway without issue.
The risk flying in this location is that I was flying in a steep and very narrow canyon. There was a real risk that GPS signal could be lost. I had a clear line of site to the drone, and experience flying without GPS, so I knew I could land the drone safely if I experienced GPS loss. I also rated the risk flying here as moderate to high.
Example #4: MAP
Tai Po Water Front Pier, Hong Kong:
This is the location I talked about in my post above. After several attempts at calibration I was not successful. There was significant signal interference, crowds of people, two kites in the air, and two drones already in the air. There were also birds nearby, and I was very close to urban and industrial areas. I did not continue my attempts to calibrate my drone, I did not fly here. I rated the risk of flying here to be very high.
Example #5: MAP
Tai Mei Tuk, Hong Kong
On a separate occasion I was flying at this location a little north of the map marker. I was standing on a short pier. I had calibrated my drone without issue, but kept getting "compass interference" when setting it down. Eventually...rather than hand-launching... I decided to set the drone down...and launch it before the warning appeared. My drone immediately signaled GPS loss, and I found myself fighting a slight breeze in order to maintain control. Being on a short pier...I wasn't sure I would have much room for error when landing. After a few minutes of "Sweating it out", The drone regained GPS, and rather than landing, I continued to fly it. I was able to land it without issue.
Risk assessment: I was flying the drone over water, in a rural area with line of site and unobstructed view. I would have rated my risk of flying here as low. My mistake was probably trying to bypass the warning.
The fact of the matter, is that GPS or signal loss can happen unexpectedly and for no apparent reason. Even in seemingly "low risk" situations.