[postgis-users] Area and scale completely wrong

[Johan Wehtje]

How would I check this.

On 11/14/05, strk at refractions.net <strk at refractions.net> wrote:
>
> None of the measurement functions in postgis take projection into
> account. Computations are purely cartesian. Can it be that the
> input (shapefile) is really in decimal degrees and it is transformed
> (reprojected) to GDA94/MGA84 during ARC load ?
>
> --strk;
>
> On Mon, Nov 14, 2005 at 08:25:54PM +1100, Johan Wehtje wrote:
> > isvalid returns true, envelope gives results that make no sense outside
> of
> > the world of Lilliput.
> >
> > I had some of the same maps but with MGA 84 zone 56 datum, and they have
> > loaded no problem, I therefore think that the definition for the GDA94
> > spatial refernces must be out of whack, because the shape files have no
> > problem when loaded in arc view in either GDA 94 or MGA 84.
> >
> > The GDA 94 shapes have attached xml files, and the pojection string in
> these
> > does look the same as the proj4 text in the Spatial Refsys table, so I
> don't
> > know what is going wrong.
> >
> > Cheers
> > (and thanks your help)
> > Johan Wehtje
> >
> > On 11/14/05, strk at refractions.net <strk at refractions.net> wrote:
> > >
> > > Does the area computation give acceptable value for the geometry's
> > > envelope ? [ select area(envelope(the_geom)) ] ?
> > > Is the geometry valid ? [ select isvalid(the_geom) ] ?
> > >
> > > --strk;
> > >
> > > On Mon, Nov 14, 2005 at 07:50:10PM +1100, Johan Wehtje wrote:
> > > > I am using Postgis 1.0.4 on Win XP with Postgresql 8.1.
> > > >
> > > > I had some Shape files that have Land Parcels , they use GDA94 Zone
> 56 (
> > > > srid 28356) , and I used shp2pgsql to load them into my DB.
> > > >
> > > > But when I an area calculation I get results that are completely
> wrong ,
> > > and
> > > > when i add a Postgis layer using QGIS the scale would have me
> beleive
> > > that
> > > > the entire shire (which about 80 km accross) is about 2 cm.
> > > >
> > > > I have tried this about twenty times, on 2 different machines - and
> the
> > > > results are the same. i have verified the original shape files in
> > > Arcview,
> > > > and when they are laoded the scale and area are all what they should
> be.
> > > >
> > > > What am I doing wrong?
> > > >
> > > > Johan Wehtje
> > >
> > > > _______________________________________________
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> > > > postgis-users at postgis.refractions.net
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> > >
> > >
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