[Proj] ITRF [Re: Google Earth Accuracy]

Duncan Agnew dagnew at ucsd.edu
Wed Dec 3 22:16:56 UTC 2008

Since there can be confusion about this, I append the "official" answer  
from the keepers of ITRF, found at
ftp://itrf.ensg.ign.fr/pub/itrf/WGS84.TXT. Note that:

1. There is more than one realization of WGS-84 (see below).

2. WGS-84 is not a projection, but a global datum.

3. As always, in the end you need to keep in mind who defines things:  
for WGS, the US DoD; for ITRF, the
scientific community, through the International Earth Rotation Service;  
and for regional datums,
whatever authority is in charge (NGS for NAD; Eurographics for ETRS  
[the European system] and so on).

4. If you are working to better than 10 cm, you should be looking for  
transformations from ITRF to your
local (national) datum--and these transformations *will* be  
time-dependent, or at least had better be,
since otherwise cm-level precision is meaningless.

Duncan Agnew

  ITRS and WGS84
  Last update : 2007-10-11

In general the ITRS (and its realizations ITRFyy) are identical to WGS84
at one meter level.

Meanwhile there are two types of WGS84 realization:

- old realization based on U.S. Navy Navigation Satellite System,
   commonly known as DOPPLER Transit, and provided station coordinates
   with accuracies of about one meter. With respect to this realization
   we published, some years ago, transformation parameters between
   ITRF90 and this Doppler realized system:

Parameters from ITRF90 to WGS84-Doppler realized system

              T1      T2      T3       D         R1       R2        R3
UNITS -----> (m)     (m)     (m)     (ppm)     (")       (")       (")

             0.060   -0.517  -0.223  -0.011    0.0183  -0.0003    0.0070

- New realizations of WGS84 based on GPS data, such as WGS84(G730, G873
   and G1150). These new WGS84 realizations are coincident with ITRF at
   about 10-centimeter level. For these realizations there are no  
   transformation parameters. This means that one can consider that ITRF
   coordinates are also expressed in WGS84 at 10 cm level.

   For more information about WGS84 realizations, see
   U.S. National Imagery and Mapping Agency, "Department of
   Defense World Geodetic System 1984 -- Its Definition and
   Relationships with Local Geodetic Systems."  NIMA Technical
   Report 8350.2 3rd release, St. Louis, MO, 23 June 2004.

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