[CSRCMAIL-1] CSRS Epoch 2004.0

[Paul Jamason]

Author: Paul Jamason and Yehuda Bock
Subject: CSRS Epoch 2004.0 Endorsed by NGS

In coordination with NGS, the CSRC has released a new epoch date (2004.0)
for coordinates of the continuous GPS stations in California that make up
the backbone of the California Spatial Reference System (CSRS).
Coordinates are provided in ITRF2000 and NAD83. CSRS Epoch 2004.0 is a
four-dimensional datum, providing both horizontal and vertical velocities
that allow the user to access epoch 2004.0 from any other date. This epoch
date replaces CSRC Epoch 2003.5 announced in CSRCMAIL #74 on 3 October
2003, and includes site deformation due to the 22 December 2003 Mw=6.5 San
Simeon earthquake in central California.

The time-varying nature of the coordinates are quantified using modeled
daily position time series computed by SOPAC/CSRC and updated on a weekly
basis (see http://sopac.ucsd.edu/coordinates/ or
http://csrc.ucsd.edu/cgi-bin/sector.cgi for access to the SECTOR utility,
and http://csrc.ucsd.edu/processing/refinedModelDoc.html for more detailed
information on this procedure).

CSRS Epoch 2004.0 is based on the modeled position time series, up to and
including 27 March 2004 (time series can be viewed at
http://csrc.ucsd.edu/cgi-bin/refinedJavaTimeSeries.cgi).  The
three-dimensional coordinates and velocities of continuous GPS stations at
this epoch define the reference frame for CSRS. Conversion to any other
epoch can be performed using the station velocities. That is, a user can
always transform coordinates to this date, using the station velocities.

To convert to NAD83 at any epoch other than CSRS Epoch 2004.0, we
recommend that the user apply the velocities to the ITRF2000 coordinates,
and then convert to NAD83 using the NGS-sanctioned transformation
parameters. CSRC will provide an interactive utility to perform this
function.

CSRS Epoch 2004.0 coordinates and velocities for all stations are
available in a single text file at
http://csrc.ucsd.edu/input/csrc/csrsEpoch2004_0.txt, or individually from
the CSRC Data Portal (http://csrc.ucsd.edu/cdp).

Important Note: The underlying daily position time series model is updated
on a weekly basis and provided through the Sector utility. Therefore, the
values obtained for this epoch using SECTOR may differ slightly (at the
few millimeter level) from CSRS Epoch 2004.0 (assuming there are no large
earthquakes). Velocities for stations with less than one year of data have
not yet been computed (and are given as 0.0000). The coordinate values for
these stations computed by Sector will differ at an increasing rate as we
collect more data and refine the model. However, the CSRS Epoch 2004.0 is
defined as the coordinates (and velocities) computed by CSRC on 14 April
2004 using data up to an including 27 March 2004, and will not change.

Please contact the CSRC webmaster <webmaster at csrc.ucsd.edu> if you have
any questions.