On Thu, Jul 17, 2014 at 09:19:43AM -0500, Bill Gee wrote:
> I am struggling with a couple of survey shots that Therion is not
> interpretting correctly. It might be a bug in how Therion averages forward
> and backward compass when the readings are near 360 and 180 degrees.
There's a bit of a gotcha when averaging compass readings, due to the
wrap-around at 360/0 degrees.
For example, if we try to average two readings: 359 and 003, then the
correct answer (at least assuming we believe them to be equally
accurate) would be 001. But if we naively sum and divide by the number
of readings, we get (359+003)/2 = 362/2 = 181.
The same issues affects averaging forward and back sights - the only
different is that the back sights get altered by 180 degrees before
averaging.
I've not tried to find where in therion this is implemented, but it
appears to explain what you are seeing here - assuming therion subtracts
180 from the backcompass, it would calculate (359 + (181 - 180)) / 2 =
180 for the first case, and (359.5 + (180 - 180)) / 2 = 179.75 for the
second.
> One of the shots (B11-B12) has compass readings of exactly 0 and 180. This
> shot displays correctly on the map.
In that care, the average would be (0 + (180 - 180)) / 2 = 0, so this
case also fits my hypothesis.
Survex is careful to get these cases right, but unfortunately it seems
that when therion uses Survex to process the centreline data, it does
the backreading averaging itself before it passes data to Survex, so
that doesn't provide a way to work around this problem.
You could put the centreline data in a .svx file, and tell therion to
process that and use the .3d file.
Cheers,
Olly
