Thread: expression used to calculate epicentral distance in SAC

Started: 2014-03-25 23:53:58
Last activity: 2014-03-28 22:40:21
Topics: SAC Help
Hi All,

If the Station Coordinate headers (STLA STLO) and Epicenter Coordinate
headers (EVLA EVLO) are defined in a SAC file, SAC computes the
epicentral distance (DIST) automatically. I would like to know the
mathematical expression and the projection system SAC uses to compute
this distance. I will be really grateful if someone can provide a link
or a reference or the actual expression itself.

Thanks.

Avinash

  • Hi Avinash,

    I believe it uses the Vincenty's formula:

    http://en.wikipedia.org/wiki/Vincenty's_formulae

    Best,
    Weisen


    On Tue, Mar 25, 2014 at 5:53 PM, Avinash Nayak <avinash07guddu<at>gmail.com>wrote:

    Hi All,

    If the Station Coordinate headers (STLA STLO) and Epicenter Coordinate
    headers (EVLA EVLO) are defined in a SAC file, SAC computes the
    epicentral distance (DIST) automatically. I would like to know the
    mathematical expression and the projection system SAC uses to compute
    this distance. I will be really grateful if someone can provide a link
    or a reference or the actual expression itself.

    Thanks.

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




    --
    Graduate Research Assistant
    Center for Imaging the Earth's Interior, Department of Physics
    University of Colorado at Boulder
    http://ciei.colorado.edu/~weisen

  • Hi,

    In my "old" manual i find:
    -------
    Note: calculations of DIST, AZ, BAZ, and GCARC are based upon

    the reference spheroid of 1968 and are defined by the major radius (RAD) and the flattening (FL). DIST is computed by Rudoe’s formula given in GEODESY, section 2.15(b).

    --------

    But i do not check if it formula has been change in the last versions.

    By the way:
    I always ask me: what is the reason, why not use the WGS-84 ellipsoid,
    so, in this way SAC could be compatible with default GMT(4,5) ellipsoid.

    Cheers,

    * * * * * * * * * * * * * * * * * * * * * * *
    Milton P. PLASENCIA LINARES

    Centro di Ricerche Sismologiche (CRS)
    OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

    Borgo Grotta Gigante 42/C
    (34010) Sgonico - Trieste - Italia
    Tel: +39 040 2140 141 (Udine)
    Tel: +39 040 2140 256 (Trieste)
    Cel.: +39 331 6481 935

    E-mail: mplasencia<at>inogs.it

    ASAIN (Antarctic Seismographic Argentinean Italian Network)
    *********************************











    On Mar 26, 2014, at 12:53 AM, Avinash Nayak <avinash07guddu<at>gmail.com> wrote:

    Hi All,

    If the Station Coordinate headers (STLA STLO) and Epicenter Coordinate
    headers (EVLA EVLO) are defined in a SAC file, SAC computes the
    epicentral distance (DIST) automatically. I would like to know the
    mathematical expression and the projection system SAC uses to compute
    this distance. I will be really grateful if someone can provide a link
    or a reference or the actual expression itself.

    Thanks.

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


    • Milton,

      We are still using Rudoe's Formula. Please see
      http://www.iris.washington.edu/pipermail/sac-help/2013-November/001592.html
      We might change to a more robust formulation.

      Changing the ellipsoid is not difficult and we have contemplated making this configurable (it will probably be done soon, maybe the next release).

      Brian Savage

      On Mar 26, 2014, at 2:28 AM, Milton Plasencia wrote:

      Hi,

      In my "old" manual i find:
      -------
      Note: calculations of DIST, AZ, BAZ, and GCARC are based upon

      the reference spheroid of 1968 and are defined by the major radius (RAD) and the flattening (FL). DIST is computed by Rudoe’s formula given in GEODESY, section 2.15(b).

      --------

      But i do not check if it formula has been change in the last versions.

      By the way:
      I always ask me: what is the reason, why not use the WGS-84 ellipsoid,
      so, in this way SAC could be compatible with default GMT(4,5) ellipsoid.

      Cheers,

      * * * * * * * * * * * * * * * * * * * * * * *
      Milton P. PLASENCIA LINARES

      Centro di Ricerche Sismologiche (CRS)
      OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

      Borgo Grotta Gigante 42/C
      (34010) Sgonico - Trieste - Italia
      Tel: +39 040 2140 141 (Udine)
      Tel: +39 040 2140 256 (Trieste)
      Cel.: +39 331 6481 935

      E-mail: mplasencia<at>inogs.it

      ASAIN (Antarctic Seismographic Argentinean Italian Network)
      *********************************











      On Mar 26, 2014, at 12:53 AM, Avinash Nayak <avinash07guddu<at>gmail.com> wrote:

      Hi All,

      If the Station Coordinate headers (STLA STLO) and Epicenter Coordinate
      headers (EVLA EVLO) are defined in a SAC file, SAC computes the
      epicentral distance (DIST) automatically. I would like to know the
      mathematical expression and the projection system SAC uses to compute
      this distance. I will be really grateful if someone can provide a link
      or a reference or the actual expression itself.

      Thanks.

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

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



      • Brian,

        Excellent news!!

        I think it update will be very important for the SAC user community.

        Cheers,

        Milton P. Plasencia Linares
        mpplasencia<at>gmail.com



        On Mar 28, 2014, at 3:06 PM, Brian Savage <savage<at>uri.edu> wrote:

        Milton,

        We are still using Rudoe's Formula. Please see
        http://www.iris.washington.edu/pipermail/sac-help/2013-November/001592.html
        We might change to a more robust formulation.

        Changing the ellipsoid is not difficult and we have contemplated making this configurable (it will probably be done soon, maybe the next release).

        Brian Savage

        On Mar 26, 2014, at 2:28 AM, Milton Plasencia wrote:

        Hi,

        In my "old" manual i find:
        -------
        Note: calculations of DIST, AZ, BAZ, and GCARC are based upon

        the reference spheroid of 1968 and are defined by the major radius (RAD) and the flattening (FL). DIST is computed by Rudoe’s formula given in GEODESY, section 2.15(b).

        --------

        But i do not check if it formula has been change in the last versions.

        By the way:
        I always ask me: what is the reason, why not use the WGS-84 ellipsoid,
        so, in this way SAC could be compatible with default GMT(4,5) ellipsoid.

        Cheers,

        * * * * * * * * * * * * * * * * * * * * * * *
        Milton P. PLASENCIA LINARES

        Centro di Ricerche Sismologiche (CRS)
        OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

        Borgo Grotta Gigante 42/C
        (34010) Sgonico - Trieste - Italia
        Tel: +39 040 2140 141 (Udine)
        Tel: +39 040 2140 256 (Trieste)
        Cel.: +39 331 6481 935

        E-mail: mplasencia<at>inogs.it

        ASAIN (Antarctic Seismographic Argentinean Italian Network)
        *********************************











        On Mar 26, 2014, at 12:53 AM, Avinash Nayak <avinash07guddu<at>gmail.com> wrote:

        Hi All,

        If the Station Coordinate headers (STLA STLO) and Epicenter Coordinate
        headers (EVLA EVLO) are defined in a SAC file, SAC computes the
        epicentral distance (DIST) automatically. I would like to know the
        mathematical expression and the projection system SAC uses to compute
        this distance. I will be really grateful if someone can provide a link
        or a reference or the actual expression itself.

        Thanks.

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

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




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