[Proj] What about datum shift via direct projection?

Gerald I. Evenden geraldi.evenden at gmail.com
Mon Dec 15 20:47:17 EST 2008

On Monday 15 December 2008 3:53:24 pm ndzinn at comcast.net wrote:
> Mikael,
> Thanks for these references on direct projection.   Weird, yes, in the
> sense of unusual, but clever and useful for the reasons stated by the
> authors.  
> Regarding the least-squares adjustment for the new CM, scale on CM and
> false coordinates, linearization of the TM algorithm by Taylor expansion
> must be exceptionally ugly, but this can be simply accomplished numerically
> to achieve the same results.

>etc, etc.

I frankly do not see what is so clever about it.  You can rubber sheet any 2D 
surface onto another---even Mickey Mouse's face.

I did basically the same thing with MAPGEN.  MAPGEN had no idea what 
projection was involve as the "projection" operation in MAPGEN was carried 
out with a polynomial approximation of the projection.  Works fairly well for 
large scale maps and is generally faster than cranking zillions of point 
though a full precision TM projection.

The only problem is that one still needs all the parts to generate the 
approximating rubber sheet.  And an added problem: keeping an audit trail for 
others to know what you did.  Also, do others have your magic rubber sheet 
evaluator or inverse operation capability?

Sorry, I have reservations about all this.

The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist

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