Sam Wormley tells us why all this works fine.
You only want one reference and the DGPS reference should be
the same as the reference system of the Global Positioning System
(WGS84). Now I have to convince you that WGS84 and NAD83 are the
same.
Parameter
Notation Units
GRS 80(NAD 83) WGS 84
____________________________________________________________________________
Semimajor axis
a
m
6378137
6378137
Angular velocity
w rad/s
7292115E-11 7292115E-11
Gravitational Const. GM
m^3/s^2 3986005E8
3986005E8
Dynamic form Factor
unnormalized form
J2
108263E-8
normalized form
C2,0
-484.16685E-6
Semiminor axis
b m
6356752.3141 6356752.3142
Eccentricity squared e^2
0.00669438002290 0.00669437999013
Flattening
f
0.00335281068118 0.00335281066474
Reciprocal flattening f^-1
298.257222101 298.257223563
Polar radius of curv
c m
6399593.6259 6399593.6258
Not much difference! (and so having DGPS data referenced to NAD 83 will work just fine.)
Ref: North American Datum of 1983
NOAA Professional Paper NOS 2
Charles R. Schwarz, Editor
U.S. Department of Commerce
The DGPS algorithms don't care what datum you've set on your GPS receiver... that is strictly display and interface FROM your receiver to outside uses such as driving the map input of StreetAtlas 6 or some other map program.. Everything will work fine. That's all the result of good system engineering... user can't screw it up!
Regards,
-Sam Wormley