Started: 2013-03-20 21:05:31
Last activity: 2013-03-20 21:27:01
Topics:
Sheila Peacock
2013-03-20 21:05:31
Dear all,

can someone tell me whether there is a correction for earth
ellipticity when the epicentre-station distance and
azimuth are worked out by SAC for various purposes?

Thanks.

Sheila Peacock
Blacknest.

• Arthur Snoke
2013-03-20 17:23:47
Sheila,

Yes there is a correction. Assuming you have the source code, you can
look at src/ucf/distaz.c. Here is a clip from the comments:

* @note Calculations are based upon the reference spheroid of 1968 and
* are defined by the major radius (RAD) and the flattening (FL).
*
* @note Distance is computed using Rudoe's forumula given in GEODESY,
* secontion 2.15(b).
* (There is some numerical problem with the following formulae.
* If the station is in the southern hemisphere and the event in
* in the northern, these equations give the longer, not the
* shorter distance between the two locations. Since the
* equations are fairly messy, the simplist solution is to reverse
* the meanings of the two locations for this case.)
*
* @note Geodesy, G. Bomford, Carlendon Press, 4th ed, 1980, Section
2.15(b)
* page 121
*
* @bug Documentation for computing distance and azimumth should be added
*
* @bug Variables here are declared static, which is really not what they
* should be declared. The actual values are difficult to identify
* what they are referring to as well.
*

This method is also an option within GMT.

Arthur

On Wed, 20 Mar 2013, Sheila Peacock wrote:

Dear all,

can someone tell me whether there is a correction for earth
ellipticity when the epicentre-station distance and
azimuth are worked out by SAC for various purposes?

Thanks.

Sheila Peacock
Blacknest.

• George Helffrich
2013-03-20 21:27:01
Dear All -

According to its documentation, SAC uses Rudoe's formula (from the book Geodesy, section 2.15(b)) based on a 1968 reference spheroid of 6378.16 km and flattening factor of 0.00335293.

On 20 Mar 2013, at 14:05, Sheila Peacock wrote:

Dear all,

can someone tell me whether there is a correction for earth
ellipticity when the epicentre-station distance and
azimuth are worked out by SAC for various purposes?

Thanks.

Sheila Peacock
Blacknest.
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George Helffrich
george.helffrich<at>bris.ac.uk

19:32:49 v.1e40bebe