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

[Frederik Tilmann]

Dear Valerie

as far as I know your interpretation is essentially correct except that I 
am not entirely clear with what you mean by
>    * 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).

If you think of each component as a vector then cmpinc is the angle the 
vector makes with vertical up, and cmpaz is the azimuth clockwise from 
north of the horizontal projection of the component vector. It does not 
make sense to talk about the cmpinc or cmpaz vector. For cmpinc=0 or 
cmpinc=180 the value of cmpaz is irrelevant, of course. As far as I know 
nowhere in SAC is cmpinc actually used except to check whether components 
are horizontal for application of the "rot to gcp" command.

A few years back I wrote a SAC command to rotate an arbitrarily oriented 
sensor (3 components needed to be perpendicular) into either VRT, ZNE, or 
LQT system, and this command used cmpinc. This used the extern mechanism of 
sac2000 but I am not sure whether this way of loading up user-defined 
commands works with the more recent versions of sac. If you are interested 
I can send you the source.

Regards
Frederik








On Oct 7 2008, Val Zimmer wrote:

>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
>_______________________________________________
>sac-help mailing list
>sac-help at iris.washington.edu
>http://www.iris.washington.edu/mailman/listinfo/sac-help
>

-- 
===============
Frederik Tilmann
Bullard Laboratories           Tel. +44 1223 765545
Department of Earth Sciences   Fax. +44 1223 360779
University of Cambridge        email: tilmann at esc.cam.ac.uk
Madingley Road
Cambridge CB3 0EZ
UK