Spectra Precision Survey Pro CE v3.60 GPS User Manual - Manual (Page 9)

GPS Coordinates
5
(ITRF). Because the earth’s center of mass and spin axis drift over
time, you will often see the WGS84 datum followed by brackets
(1996.0). The date in the brackets indicates the epoch defining the
datum.
This is all quite confusing. Fortunately, for most RTK GPS
applications, you do not need to worry about these WGS84
differences. The significant part of the datum difference is a shift,
and you correct this when you specify the GPS base position. The
other part of the datum difference is the small rotation of the axes.
These rotations are small enough to ignore except for the most precise
first order applications.
If your Survey Pro job requires a local datum in one epoch of WGS84
and the WGS84 datum in a different epoch, you can setup a seven-
parameter similarity transformation. For the transformation
parameters of any epoch of WGS84 and for a more detailed
description of the similarity transformation and WGS84, see NGS
web site
2 .
• High Accuracy Reference Network (HARN)
In the United States, the bulk of the measurements used to establish
NAD83 were conventional. These measurements contain slight
systematic errors that conflict with GPS measurements, which are
more precise over long distances. To address this problem in the
U.S.A., in 1988 the National Geodetic Survey (NGS) began to update
NAD83 coordinate datums with HARN GPS surveys on a state-by-
state basis. These HARN surveys determined small (< 5_cm)
corrections to the location of A and B order control monuments across
the states.
Survey Pro performs a grid transformation for HARN networks in the
United States using the NADCON datum files in *.DGF format.
Note: To use a grid datum, you must have the pair of *.dgf files for
latitude and longitude shift the Disk\Geodata directory.
2 Snay, R. How CORS Positions and Velocities Were Derived.
http://www.ngs.noaa.gov/CORS/Derivation.html Appendix B.