Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual User Manual

Page 240

User’s Manual – GPS Mode

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(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

• 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.

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

Snay, R. How CORS Positions and Velocities Were Derived.

http://www.ngs.noaa.gov/CORS/Derivation.html

Appendix B.

http://www.ngs.noaa.gov/NationalReadjustment/Items/The%20nation

al%20readjustment.html