[Proj] A datum question if I may

Clifford J Mugnier cjmce at lsu.edu
Tue Oct 25 20:00:58 EDT 2005

NAD83 is going toward a merge with the ITRFXX (International Terrestrial
Reference Frame of some year) and WGS84 is already on the ITRFXX.  These
are reference frames with respect to DATUMS.  The datums are the same
within centimeters on the ground.  Big deal in Geodesy and precise Land
Surveys - zippo for GIS.

The minor difference in the 14th decimal place is with respect to the
difference in the WGS84 ellipsoid and the GRS80 (Geodetic Reference System
1980) ellipsoid because two different formulae are used to define the
respective gravity fields.  Since "b," "1/f" reciprocal of flattening, etc.
are derived quantities, the derivations differ slightly.  There is ZERO
practical difference.

C. Mugnier
On 10/25/05, Hersman, William <willy.hersman at conocophillips.com> wrote:
> Is there a difference between a NAD27->NAD83 conversion and a
> NAD27->WGS84 conversion ?  Much ?


PROJ.4's datum conversion logic considers NAD83 and
WGS84 equivelent since the differences are so minor.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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