[Proj] Fwd: [MetaCRS] Common SQLite-based dictionaries

Huw James huwejames at gmail.com
Sun Aug 2 11:02:08 EST 2015

Dear Even,

people working today with CRS tend to assume that there is one set of
parameters to perform a particular coordinate conversion.

In the real world we have coordinates that were derived from some Latitude
& Longitude on some datum that used datum transformations, ellipsoid
definitions and numerical precision that are far different from those used
today. The parameters and precision that were used were often the best
available at the time which often preceded any satellite derive knowledge.

There are many assets around the world whose coordinates and ownership are
defined by coordinates that would change if current EPSG or other
"standard" parameters were used. This would include moving valuable assets
across national and private property boundaries. Disallowing customization
of datum and projection parameters would cause no end of confusion in the
real world. By saying that coordinates based on custom parameters for ED50
& UTM can be supported by defining a new custom datum for UTM and ED50 is
not useful since the link between ED50 and the new pseudoED50 and the link
between UTM and the new pseudoUTM are both lost and it is unlikely that
some destination computer would have definitions or code for these new
pseudo CRS definitions.

I think it is better to define the customized CRS as ED50 & UTM with its
non-standard definitions. Systems that do not support historical CRS
conversions will not be adopted by users that need to deal with existing

Today's users tend to think that coordinates can be defined to 1 mm
precision or less. This is a recent capability. When surveying and
navigation accuracy was at 50 m it seemed foolish to define parameters or
coordinate conversion software with sub-meter precision.

Huw James
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.maptools.org/pipermail/proj/attachments/20150802/a0dd42c8/attachment.htm 

More information about the Proj mailing list