Here I will point out how ITRF coordinates and velocities given in the NGS data base for CORS will change during the upcoming readjustment. There will be three causes of change. They are:

1. change due to a change of coordinate system from ITRF2000 to ITRF2005

2. change due to a change in the initial epoch used in presenting results

3. change due to computing more accurate coordinates and velocities relative to the coordinate system being used.

As I pointed out previously on the main board there will be coordinate changes under 1. due to small changes in the location of the origin of the two coordinate systems and a scale difference of the order of a fraction of a part per billion. The change due to coordinate system change can be expected to be no more than 1.5 cm. The two coordinate systems will not be rotated relative to one another.

In the data base now ITRF00 initial epoch coordinates are given as of 1997.0. If the initial epoch ITRF05 coordinates for a CORS in the readjustment are given in the data base at some other epoch than 1997.0 the values given will, of course, be different because of movement of the CORS over time relative to the ITRF05 coordinate system. The size of this difference will depend on the size of the ITRF05 velocity multiplied by the time difference between 1997 and the initial epoch time chosen by NGS for the readjusted ITRF05 coordinates. This initial epoch can be expected to be somewhere in the range of 2000.0 and 2000.5. NGS will no doubt be making a decision very soon. The published difference of initial epoch values of coordinates could be quite large, in California 10s of centimeters and even in the eastern US over 10 cm. But these changes will be of no consequence in the surveyors use of coordinates provided comparisons are made after correcting to a common effort.

The final cause for change will be changes in initial epoch coordinates and velocities due to additional observations made since ITRF00 coordinates were computed and improved reduction techniques. I would not expect initial epoch coordinates to change from this source by more than 1 or 2 cm. In most cases I would not expect velocities to change by more than 1 or 2 mm/yr.


Modified By Mister Geodesist on 11/22/2006 at 5:04 PM