[Therion] Radiolocation Fixed Points

[Tarquin Wilton-Jones@test.com]

> 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.

>From the specs, it sounds like roughly grade 2 compass (5°), reasonably
accurate depth (30 cm), but using gyroscope and accelerometer to measure
motion is not exactly accurate, at least in most consumer grade
equipment. But I would love to hear otherwise!

If you set the "sd" options correctly for the device, and set the
appropriate standard deviations for the radio location fixes, then
Therion should be able to sort out the error distribution accordingly.
sd for a radio location is likely to be quite large, but actually not as
bad as you might expect.

Radio location "may be able to locate the horizontal position of an
underground point to within just 1.5% of the depth below ground (about 1
metre per 67 metres of depth), and the depth to about 5% of its actual
depth, depending on how level the surface is". But that is in addition
to whatever error there is in locating the point on the surface where
the radio location antenna was used (ie. if you survey to it at grade 5,
or locate it with a GPS that has a 5x5x15 m error - fairly good for a
GPS, no matter what yours might like to claim). Both of those errors
would need to be put into the standard deviation, to get it right.

eg. for a passage 200 m below surface:
1.5% * 200 = 3
5% * 200 = 10
3 + 5 = 8
10 + 15 = 25
fix sumpstart 123 123 123 8 8 25 # 8 m easting, 8 m northing, 25 m alt