[Therion] Radiolocation Fixed Points

[Tarquin Wilton-Jones@test.com]

On 25/08/2022 20:16, Tarquin Wilton-Jones--- via Therion wrote:
>> I am only suggesting removing one of the radiolocation points because
>> your siphon survey sounds like it's fairly accurate, and as tarquin says
>> not likely to be causing the error. I would think the accuracy of the
>> radiolocation could be the weak link, but i'm no expert with radiolocation.
> 
> Depends. I would say something that uses gyroscopes to detect its
> position, is likely to be quite inaccurate, but that really depends on
> the quality of the device. I know Andrew has worked with some supremely
> accurate devices, but in this case, I would consider it quite suspect.

Did some more looking into it. The manufacturers do not state the
accuracy of their inertial sensors (gyroscopes and accelerometers). This
is a summary of what I found:

This particular one works using GPS when in contact with the surface,
then gyroscopes, accelerometers, compass and depth gauge underground or
underwater. Its design seems to be aimed at the use of scooters, rather
than being handheld. The compass has only 3-5° accuracy (unless
specifically calibrated), and the depth gauge has 0.3m accuracy. Part of
its operation uses a speed sensor which detects water flow, so its
results vary depending on the current within the cave, and its accuracy
is 2% when moving at scooter speed (and likely far worse at slow
speeds). The main accuracy limitation is the quality of the inertial
sensors, which will become progressively worse over time, so the
accuracy decays over longer dives. Even a high quality device is
expected to lose 50 m of accuracy within 15 minutes, and a poor one
within just 10 seconds [taken from a forum thread about this exact
problem with this device]. The manufacturer does not state the quality
of the sensors or their use of them. The equivalent cave surveying grade
is therefore very hard to estimate, and probably equates to grade 2 at best.

It would be interesting to know how well the device actually performs in
a test without GPS, comparing a disto survey with an underwater dive
survey done using an ENC3.