What the quality tags mean

The tags

Why a single clock is not enough

The pen's own timestamp looks like a clock and is not one. On the hardware here it advances a flat 4.166 ms per delivered packet whatever the elapsed time, runs at 0.673 of real time, and resynchronises to real time at every contact transition. It implies a report rate of 240 a second; the real rate, measured against the host's own clock, is 161.6.

So a recording carrying only that timestamp cannot date a reading in real time, and speed computed from it is speed per counter-second. Recordings at format version 5 and above carry a second timestamp taken from the host, and that one is a clock.

It also explains the hover tag. The recorder used to decide how old an airborne reading was using the pen's counter, which runs slow — so readings were discarded that should have been kept, and the approach to a landing came out short. Only format version 6 ages the approach on the host clock.

Which recordings want drawing again

This is a list of work, not a warning about the data that is here. Each of these was recorded before some part of the recorder was fixed, and each is worth doing again.

What the format versions added

A recording carries what its version carried and no more. The absence is real: a version-two recording genuinely has no height, and anything that filled one in would be publishing a number nobody measured. The format in full.