# 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:
Avinash
2014-03-25 23:53:58
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

• Weisen Shen
2014-03-26 03:25:18
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

--
Center for Imaging the Earth's Interior, Department of Physics

• Milton Plasencia
2014-03-26 14:28:01
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

• Brian Savage
2014-03-28 17:06:56
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

• Milton Plasencia
2014-03-28 22:40:21
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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