[David Hoover: NAD Conversions]

[Nick Reddingius]

David Hoover writes in part:

>  I have some concerns about explaining datum conversions. 
   ....  stuff omitted  ....
>  For example, in the geographic coordinate system, 90 degrees will  
>  still be straight from the center of the earth to the north pole and 
>  0 degrees will be perpendicular to that, and datum shifts will not 
>  change that. 

The problem is, the Earth isn't a sphere; it isn't even an ellipsoid.
The Earth has neither smooth surface nor uniform density.  As a result,
surveyors, who have to measure real-world phenomena with real-world
instruments make certain assumptions about the ellipsoid that will best
approximate the geoid.  NAD27 is one of those assumptions and NAD83 is
another.  Being based on satellite measurements, the latter has its
center of coordinates at the center of gravity of the Earth, the
former does not.

Thus while 90 degrees is "up" for both, the two axes won't "cut through
the surface of the planet" at the same spot.  (Whether either point is
the North Pole or not depends to some extent on how you define the
North Pole: inertial axis, axis around which the precession takes place,
or magnetic.)  All coordinates, therefore, whether Lat/Long or UTM, will
change when the datum is changed.

If you are mathematically inclined, you can find this all explained in
gory detail in a reference text on geodesy, or perhaps you can find the
August 1993 issue of GIS World, which has on page 26 a pretty accessible
article with ominous title "Can the GIS Community Deal with a Geoid's
Mathematical Complexity?"  Seems like lots of GIS users are a bit hazy
on this subject.....

As a corollary: you CANNOT use the GRASS Lat/Long to UTM routines to
convert NAD27 UTMs to Lat/Long using the clark66 spheroid, then convert
those Lat/Long coordinates back to UTM using the wgs84 spheroid.  Well,
of course, you can, but you will end up with useless numbers:  it will
NOT be a transformation from NAD27 to NAD83!  (There are programs from
other sources that will do the transformation, though.  I believe that
USGS proj does it, but I've not had a need to use it so I can't promise
that it does it.)

Nick
the