[SAC-HELP] Calculation of AZ and BAZ in SAC

[Arthur Snoke]

Fiona,

Brian May have given an adequate answer to your question, but perhaps you 
are asking when one should use BAZ and when one should use AZ.  Here are a 
few comments on that.

In SAC, BAZ is defined as "Station to event azimuth (degrees)", and AZ is 
defined as "Event to station azimuth (degrees)."  For receiver-fuction 
work, as for surface-wave analyses, BAZ is the one to use because it can 
be used to transfer the NS and EW horizontal components into Radial and 
Transverse -- the transformaiton should be made at the station.  For 
focal-mechanism studies, AZ shoud be used, as one wants to know the 
direction rays leave the source.  For practical purposes, the difference 
between DOST and GCARC probably is too small to worry about, but DIST is 
probably est for receiver functions.

On Mon, 18 Nov 2013, Fiona Darbyshire wrote:

> Hello,
>
> Could someone tell me a bit more about how AZ and BAZ are calculated in SAC?
>
> Up to now, e.g. when binning receiver functions, I have used BAZ and
> GCARC to group them together, and that's always seemed to work, so
> I've always assumed that the BAZ/GCARC (or DIST, I imagine) pair gives
> you a unique position from the point of interest (the station).
>
> This time, I have some data output that is expressed in AZ and DIST. I
> assumed (perhaps naively?) that this would also allow successful
> spatial grouping, but it turns out not to be the case; one can have
> the same forward azimuth for several different locations.
>
> Here's the example where I found the issue:
> Station lat, lon: 82.5033, -62.35 (ALE)
>
> Event1: lat 80.205, lon -1.091
> Event 2: lat 86.876, lon 54.373
> The two events are almost 1000km apart.
>
> Event 1: AZ 313, BAZ 72, GCARC 9, DIST 1006
> Event 2: AZ 314, BAZ 17, GCARC 9, DIST 1042
>
> I'd like to know more about the calculation, in particular because (i)
> I'd like to reassure myself that BAZ/distance does indeed give a
> unique location, and (ii) I don't understand how AZ is calculated. One
> website I looked at (Matlab) talks about rhumb lines vs great-circles;
> is that the issue? Will an AZ/distance pair always be non-unique?
> (Also most websites define back-azimuth as simply 180 opposite to
> azimuth, but I guess they aren't talking about spherical geometry
> then...)
>
> Thanks for any insight.
>
> Fiona Darbyshire.
>
> Centre de recherche GEOTOP, Université du Québec à Montréal
>
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