[postgis-users] Area and scale completely wrong

[strk at refractions.net]

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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