Spectra Precision Survey Pro v4.6 Ranger User Manual - Manual (Page 218)

User’s Manual – GPS Mode
208
(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 .
• NAD83(1986) = NAD83(1996.0, 1999.0, …)
In the same way that the control for the WGS84 datum is adjusted
over time, the control for the NAD83 datum is also periodically
updated. One motivation for the periodic readjustments of NAD83 is
to account for the relative velocities of points on the stable North
American tectonic plate and points on the moving Pacific tectonic
plate. Chris Pearson of the US NGS has written a good paper on the
history of NAD83 and the new US National Readjustment, completed
in February 2007 and identified as NAD83(NSRS 2007). This paper
can be found on the NGS web site.
3
It is therefore important to pay attention to the epoch associated with
a NAD83 control coordinate. If you are looking at an NGS data sheet
for a number of different control monuments, make sure they are all
in the same epoch of NAD83. Similarly, if you are using an OPUS
solution and an NGS control monument on the ground, pay attention
to the epoch of the datum of each position.
The good news again is that for RTK surveys, you usually do not need
to worry about these differences. The important thing is to not mix
2 Snay, R. How CORS Positions and Velocities Were Derived.
http://www.ngs.noaa.gov/CORS/Derivation.html Appendix B.
3
http://www.ngs.noaa.gov/NationalReadjustment/Items/The%20nation
al%20readjustment.html