Plus the iPhone/iPad uses GPS and GLONASS. The current GPS systems always have multiple satellites locked in. Virtually all my flying is in the mountains and with the paper maps, I can visualize where I am by comparing the topo features with the map, thus getting primary headings.

As for reciprocals, plus 2 minus 2, or or the reciprocal works for most headings. i.e., 180 +2 - 2 is 360. Doesn't work for every heading but works for most. 270 -2 + 2 is 90. The result must make sense, but I have used this method most of my flying career.

I do agree, the vertical card compass is the most useful in aviation.


The GPS system isn't a single point of failure, there are currently two fully operational systems, soon to be 3 and in a while longer 4.