[MetaCRS] Motion Vote: CS-Map RFC 2 - Redesign Datum Transformation Engine

[Frank Warmerdam]

Martin Desruisseaux wrote:
> In other words, the result of two consecutive Datum Shift is usually not 
> expressible in a single set of Bursa-Wolf parameters, except if the 
> datum shifts are only geocentric translations (no scale or rotation).

Martin,

OK,you are making me think a bit more about what I mean when I talk about
using WGS84 as a pivot datum.

I wasn't thinking the multiple shifts could be reduced to a single Bursa-Wolf
transformation, but rather that each datum shift would still occur by passing
through WGS84.

So for a rather made up example lets assume there is a translation from
Estonia 1992 to ETRS89 and a transformation from ETRS89 to WGS84, I would
like to be able to do the datum transformation from Estonia 1992 to ETRS89
by going from Estonia to ETRS89 to WGS84 and then back to ETRS89.

Thus, I would keep track of the preferred path to WGS84 for Estonia 1992
as being Estonia 1992 to ETRS89 to WGS84 and always apply them as a chain.

That is, I am willing to support chaining, but would still like to have
every transformation go through the grand-central station of WGS84 for
simplicity.

What are the downsides to using WGS84 as a pivot but with chained
transformations?

Best regards,
-- 
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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