[Antelope] how does antelope define the axis vang in the ddbatch file?

[Val Zimmer]

Hello Antelope users -

I have a dataset from a station that was NOT placed perfectly level - 
e.g. the sensor was placed on a ledge that had a little bit of tilt, 
such that Z is not perfectly up/down, and N + E have some down or upward 
component in the data.  I'm now trying to build a database from that 
data, but have yet to find a good definition of vang in the dbbatch file 
(for axis).   This is, of course, something that the original 
programmers would have defined in the source code, assuming antelope 
does something with that "axis" component (as opposed to it being merely 
a comment).

Having thought about this a while, I can think of only one good, logical 
way to define vang, although, I'm probably missing something, and would 
like to verify that this is correct. 

Here's an example from some documentation:
axis <name> <hang> <vang>       ?     ?
axis    Z            0         180         - 1     1
axis    N            0         90          - 2     1
axis    E            90         90         - 3     1

hang (horizontal angle) has a well-defined, inherent, orientation (N = 
0, E = 90, S = 180, W = 270) and an normal viewpoint (map/above view, 
with N up, E right, etc).  Ok, easy, no problem.

vang (vertical angle), however, has no inherent orientation, although we 
can infer the following things from the above example, assuming we are 
dealing with the earthquake seismology definition (e.g. +Z is up, NOT 
down like in the oilfield):  Up = 180, Down = 0.  Of course, there is no 
"normal" viewpoint when it comes to cross-sectional views, and trying to 
define one is bound to confuse and contradict, therefore, the direction 
of the vang vector *must* be relative to the hang vector AND that <vang> 
vector corresponds to apparent tilt along that vector (not 
absolute/maximum tilt for the whole instrument in whatever direction 
that happens to be). 

Therefore, I *think* the only possible way to define it would be that 
<vang> must be 0-> 180 and that the vector points in the same direction 
of the <hang> vector, and it doesn't matter whether 90 degrees is to the 
"right" or to the "left" since that totally depends on the perspective.  
Hence, it follows that:

axis  S        180    90 (not, and never 270)
axis W       180     90

axis N+ a little up          0       110
axis E + a little down     90      75

Can anyone tell me if this reasoning is correct, and if not, point me to 
some documentation with a clear definition?

Thanks for your time!
Valerie Zimmer


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