[postgis-users] Re: [Freegis-list] MySQL goes spatial

[Hubert Fröhlich]

Hi Paolo,

> Hi all.
> Does anybody have experience on the spatial extension of freegis?
> Info at:
> http://dev.mysql.com/tech-resources/articles/4.1/gis-with-mysql.html
> especially interesting would be a comparison with postGIS/GEOS.
> All the best.
> pc

no experience, but ...

.. when I read the article above, I found
<quote>
In addition to the above, there is also a need for GIS metadata, and in 
some cases for using different coordinate systems. MySQL currently 
supports a planar coordinate system. The other major coordinate system 
in use is the geocentric one, i.e. a coordinate system on the Earth's 
surface, which is not yet supported by MySQL.
</quote>

This might be quite a big practical restriction. The earth *** is not a 
planar surface *** , and lots of esp. numerical calculations, e.g.
  * area calculations, such as Area() in postgis
  * topological questions (think of functions e.g. PointOnSurface() in 
postgis)

have to respect that, otherwise your results are false. You always 
depend on a special spatial reference system (SRS) , and there are lots 
of them ... Of course, this does not hold for low accuracy and small 
areas etc, but you have to be aware and in case of doubt leave these 
calculations to an application.

(BTW: How are these cases handled in PostGIS: Does PostGIS calculate in 
a planar way or truly on the ellipsoid chosen with the SRS? I screened 
the postgis docs and the OGC specs, but I did not get the information)

Greetings,

Hubert



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Dr.-Ing. Hubert Fröhlich			
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