[Proj] Re: Campo Inchauspe to WGS84 parameters

[Noel Zinn]

Good points, Cliff, but you should caution your readers about the perils of
deriving 7-parameter shifts in "small" areas (and the use of 7-parameter
shifts so derived).  The smaller the area of the derivation the higher are
the correlations among the translations (as a group) and the rotations and
the scale (as another group).  In a small area the user should be satisfied
with deriving and using a 3-parameter shift, since the 3 orthogonal
translations are uncorrelated.  Introducing the rotations and scale to the
adjustment leads to (natural and expected) coordinate "noise" (inaccuracies)
driving big numbers for the solved parameters, which tend to countervail one
another and which are, therefore, physically meaningless.  In a small area
the 7-parameter derivation is an ill-conditioned adjustment that should be
avoided.  Of course, if you've already got a 7-parameter shift (given to
you) you need to use all 7 parameters (or you're in big trouble).  -Noel
Zinn  

-----Original Message-----
From: proj-bounces at lists.maptools.org
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Clifford J Mugnier
Sent: Friday, November 11, 2005 10:45 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Re: Campo Inchauspe to WGS84 parameters

ALL datum shifts are approximations, particularly those that only use the 3
translations.  Accuracy increases as the number of parameters increase
and/or the size of the region being transformed decreases.

One generalization that is ALWAYS correct is that EVERY datum shift will
decrease the accuracy of the original data once it is transformed to the
other datum.

EPSG-published parameters are directed to regions that are predominately
hydrocarbon-producing ... e.g. coastal Argentina.  NIMA-published
parameters are directed to regions that are potential battle zones on land
... e.g. interior AND coastal Argentina.  None of them are "exact."
Classical datums have systematic errors that change greatly over vast
regions.  Argentina is not a small country.  Consider also the many sets of
3-parameter transformations available for the Continental U.S.

For those that "cherry pick" parameters from my columns, I suggest reading
the entire column with the associated background information for the
country of interest.  The primary living authority on Argentine Geodesy is
Mr. Ruben Rodriguez, as referenced in my column.  He is a practicing
Geodesist that lives in Argentina.

Datum shifts are "starters" for general reconaissance.  When one wants the
EXACT transformation parameters for a specific area, one puts a Geodesist
on the ground to find out.  Practical surveying beats out office theory
every time.

Cliff Mugnier

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