Thread: epicentre-station distance - ellipticity

Started: 2013-03-20 21:05:31
Last activity: 2013-03-20 21:27:01
Topics: SAC Help
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



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