HowTo: Cavern output in feet

[Olly Betts]

On Tue, Mar 03, 2015 at 10:32:13AM -0600, Bill Gee wrote:
> Thanks!   I suspected there was no option to display the output in feet.  It 
> should be a simple change, so yes - let's add it to the wish list for a future 
> version.

It's been in TODO.htm since March 2002, it's just not actually something
anyone has asked about before.

> I assume cavern is converting feet to meters internally.  Converting 
> back should be easy.

I've implemented this for everything except the .err file in commit
85c0078.  It's more involved than you might think, mostly because all
the messages are translated.

There's also the question of how to decide what units to use in each
case.

For errors and warnings during processing, it seems most helpful to use
the units in force for the quantity in question at the time, so for
example if your compass readings are in grads, you'll now get a
foresight/backsight mismatch reported in grads (it actually uses the
units of the foresight instrument, which you can tell if you're perverse
enough to use an instrument working in different units for the
backsights).

For the stats at the end of processing, the units for the tape in force
at the end of processing are currently used.  That doesn't pick feet for
AllieSpringCaveSurveyData.svx as the "*units tape feet" is inside a
*begin/*end block, which is probably a sensible place for it.  So I
think a better rule is needed for this, and it needs to do something
sensible if the data is a mixture of metres and feet.  But you can at
least now stick a "*units tape feet" on the end of the main file you
process to force the stats to be in feet.

Perhaps the tape units used for the first reading processed (which is
consistent with using the first specified output coordinate system if
you specify it more than once).  That seems quite nice, but it means
there's no simple way to add something to force particular units.

Perhaps the first leg or "*units tape X", whichever comes first - its
a more complex rule, but it means you can add a dummy "*units tape X"
before any data.

I've not tried to convey units preferences to aven via the 3d file -
that wouldn't be hard to add to the file, but I'm not sure if it's what
people would actually want.  I think I'd probably rather not have aven
switch to feet and grads automatically when someone sends me a 3d file
surveyed in those - especially for the angles that's likely to be more
confusing that helpful.

> Regarding the vertical length - I assumed this was the case.  The
> resulting total is not terribly useful.  Thanks for the explanation.

The sum of the horizontal components of the legs used to be used as a
measure of cave length by some before the UIS standardised using the
total surveyed length.  It was sometimes called "True Horizontal
Distance" - you sometimes see 'THD' next to the cave length on old
surveys.

But it seems a bit unfair not to give the vertical components equal
treatment.  I'm not sure either total is actually very useful, but you
can at least compare the two to see if your cave is mostly horizontal or
vertical.

Cheers,
    Olly