[Proj] Re: Campo Inchauspe to WGS84 parameters

[Oscar van Vlijmen]

>From mr. 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.

Or use a 10-parameter Molodenksy-Badekas transformation. See EPSG coordinate
operation method 9636.
Example: the Dutch Cadastre has calculated coefficients for transformation
between Bessel 1841 and ETRS89. Rotation is around datum point Amersfoort.
See section 7.2.1 in "Het stelsel van de Rijksdriehoeksmeting en het
European Terrestrial Reference System 1989 - Gebruik en definitie"
(The RD system and the ETRS89 system - use and definitions; in Dutch
language).
http://www.rdnap.nl/download/rdnaptrans.pdf



  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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