Hi folks, I guess this could just as easily be a Survex question as a Therion question. I have GPS readings taken on a pole, with the position being given for the top of the pole. Subsequent Disto readings are taken from the bottom of the pole. The pole is brought to vertical using a spirit level. I have checked the accuracy of the spirit as being 0.125°. I have the (measured) standard deviations of the GPS positions, so I can specify the standard deviations for the location fix. What I need to work out is how to specify the standard deviation for the pole. It's not a plumb measurement, because it is not purely gravitational. So although I could use "DOWN" that is not actually accurate, since it is really a nearly-vertical measurement depending on the accuracy of the spirit level. It's really a length and clinometer reading. #length of the pole accuracy measurements sd length 0.001 metres #accuracy of the spirit level sd clino 0.125 degrees But when working with a supposedly vertical position, even the slightest error in the clinometer angle of the pole could cause the compass bearing to flip to any angle at all. But this seems extreme: sd compass 360 degrees So what would be the right way to specify it? Cheers, Tarquin
I guess this could just as easily be a Survex question as a Therion question.
Actually, it's not. In Survex terminology, it would be done like this: *sd length 0.001 metres *sd plumb 0.125 degrees gps surveystart 2.000 - DOWN That is how you are supposed to do a vertical leg with variable clinometer accuracy and length accuracy, but no compass reading (this is taken from Survex's "How do I?" article, when talking about a radio location fix). In Therion, this generates an error "invalid quantity -- plumb". sd length 0.001 metres sd plumb 0.125 degrees gps surveystart 2.000 - DOWN So how do you do the equivalent in Therion without being allowed to use "sd plumb"? Is this a bug in Therion? Does it use clino instead? Can it pass this to the underlying Survex correctly? Cheers, Tarquin
I have GPS readings taken on a pole, with the position being given for the top of the pole. Subsequent Disto readings are taken from the bottom of the pole. The pole is brought to vertical using a spirit level. I have checked the accuracy of the spirit as being 0.125°.
I have the (measured) standard deviations of the GPS positions, so I can specify the standard deviations for the location fix. What I need to work out is how to specify the standard deviation for the pole. It's not a plumb measurement, because it is not purely gravitational. So although I could use "DOWN" that is not actually accurate, since it is really a nearly-vertical measurement depending on the accuracy of the spirit level. It's really a length and clinometer reading.
#length of the pole accuracy measurements sd length 0.001 metres #accuracy of the spirit level sd clino 0.125 degrees
But when working with a supposedly vertical position, even the slightest error in the clinometer angle of the pole could cause the compass bearing to flip to any angle at all. But this seems extreme: sd compass 360 degrees
So what would be the right way to specify it?
Cheers,
Tarquin _______________________________________________ Therion mailing list Therion@speleo.sk https://mailman.speleo.sk/listinfo/therion
In Therion, this generates an error "invalid quantity -- plumb".
sd length 0.001 metres sd plumb 0.125 degrees gps surveystart 2.000 - DOWN
I wrote a bunch of tests. Therion is missing the required "sd plumb" functionality, and has no equivalent. I have filed this as: https://github.com/therion/therion/issues/445 The only workaround is to use: sd length 0.001 metres sd clino 0.125 degrees gps surveystart 2.000 - -90 but this will then generate a warning from Cavern saying that the compass bearing may not be omitted. (It will still work, however.)
On Wed, Sep 07, 2022 at 10:56:00AM +0100, Tarquin Wilton-Jones via Therion wrote:
In Therion, this generates an error "invalid quantity -- plumb".
sd length 0.001 metres sd plumb 0.125 degrees gps surveystart 2.000 - DOWN
I wrote a bunch of tests. Therion is missing the required "sd plumb" functionality, and has no equivalent. I have filed this as: https://github.com/therion/therion/issues/445
The only workaround is to use:
Another workaround is to process your centreline data directly with cavern and point therion at the resultant .3d file. However, if you have existing centreline data in therion format that would require converting it which is probably too annoying.
sd length 0.001 metres sd clino 0.125 degrees gps surveystart 2.000 - -90
but this will then generate a warning from Cavern saying that the compass bearing may not be omitted. (It will still work, however.)
Not sure what therion generates to feed to cavern in this case, but presumably this gets handled as having a compass reading of 0, so it doesn't give you the symmetric situation you want. Setting the compass SD high probably won't give you the symmetric situation either. As you concluded modelling this as a plumb is the best approach in Survex. The plumb error is assumed to be a small angle from the vertical since the main error component is assumed to be linear in the length of the leg. I'm not sure if those assumptions are entirely justified, but having the error increase with length seems appropriate as on longer plumbs the end tends to wander over a larger area and the period of oscillation for a pendulum increases with length (proportional to sqrt(length) if you assume the string is weightless, or if you assume the whole pendulum is rigid, neither of which are going to be entirely true for a plumb in a cave). It might be interesting to experiment to see what the plumb reading error actually looks like with increasing length (perhaps down a stairwell in a tall building). Anyway, the main error from your spirit level on the stick will also be a small angle from the vertical. Cheers, Olly
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Olly Betts -
Tarquin Wilton-Jones