[SAC-HELP] how does SAC define the header variable CMPINC, exactly?

[Val Zimmer]

Hello SAC 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 analyze that data, but have 
yet to find a good definition of cmpinc in the manual (all it says is  
"Component incident angle (degrees from vertical)").  CMPINC has no 
inherent orientation (like CMPAZ, from north, and looking down with 90 
to the right e.g. east), but I can think of only one good, logical way 
to define cmpinc.  Although, I'm probably missing something, and would 
like to verify that this is correct, hence the email to you all.

I can infer the following things from pulling the data from an 
earthquake seismology station (cmb.bk):

    * +Z is probably up, (not down like in the oilfield):  Up = 0, hence
      Down = 180. 
    * the other components have cmpinc = 90 (N + E)
    * Therefore, the direction of the cmpinc vector *must* be relative
      to the cmpaz vector AND that cmpinc vector corresponds to apparent
      tilt along that vector (not absolute/maximum tilt for the whole
      instrument in whatever direction that happens to be).
    * It follows other axes would have to be defined as follows:
          o DIR               CMPAZ       CMPINC
          o south                 180               90
          o west                  270               90
          o n + a little up       0                 75
          o e + a little down  90              110

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

Oh, and if any of you know how Antelope defines the <vang> vector, I'm 
also trying to figure that out (I think that Antelope's definition of 
"vertical" is different, e.g. Up = 180 and Down = 0, although, I cannot 
confirm it).

Thanks for your time!
Valerie Zimmer