My GPS computes internally using WGS-84 Datum.  The US Coast Guard DGPS signals come in using NAD-83 Datum and I want to select NAD-27 as my output Datum to my computer.  Does my GPS receiver sort all this out properly?

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!


-Sam Wormley