[mapserver-users] PostGIS layer projection

[Radim Blazek]

Hi Jeff,
thanks, yes I have read that.

But I also read that  "using srid= is optional but improves
performance" so there was a little chance that re-projection could be
done anyway if a different SRID is found on geometry.

If there is no further transformation, what difference "using srid="
does? What happens if SRID is not set on column and there are mixed
geoms' SRIDs.

I mean, where is CRS transformation done if request CRS != layer CRS
!= column CRS? I would expect calling directly PostGIS
ST_Transform(geom, request_srid), which would by pass
LAYER.PROJECTION.

Radim

On Thu, Jul 18, 2019 at 12:31 PM Jeff McKenna
<jmckenna at gatewaygeomatics.com> wrote:
>
> Hi Radim,
>
> Give the Projection document a good read specifically the "Important
> Notes" section which I believe answers your questions:
> https://mapserver.org/mapfile/projection.html#important-notes
>
> There are no specific driver rules; for something like a PostGIS
> connection you should be able to follow the "Important Notes" above.
> MapServer will assume that you have configured the driver properly (in
> the case of PostGIS, setting the SRID as well, since PostGIS requires
> that parameter to do a projection)
>
> -jeff
>
>
> --
> Jeff McKenna
> MapServer Consulting and Training Services
> https://gatewaygeomatics.com/
>
>
>
> On 2019-07-18 5:59 AM, Radim Blazek wrote:
> > Hi,
> > can you please explain how does re-projection of PostGIS layers work?
> > If there is a POSTGIS layer with DATA without "using srid=" but
> > PostGIS column has SRID defined, there is constraint for that SRID on
> > the column and geometries have SRID set, is it necessary to define
> > PROJECTION for that layer if SRID of the column is different from that
> > of mapfile?
> >
> > For example, geometry column has SRID=4326, geometries have SRID set, mapfile:
> >
> > NAME test
> >       PROJECTION
> >           "init=epsg:3857"
> >       END
> >       LAYER test
> >            CONNECTIONTYPE postgis
> >            DATA "geo FROM test"
> >       END
> > END
> >
> > if a request comes to render a map in 3857, will be geometries
> > re-projected correctly from 4326 to 3857?
> >
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