[postgis-users] Area and scale completely wrong

[strk at refractions.net]

On Mon, Nov 14, 2005 at 09:34:12PM +1100, Johan Wehtje wrote:
> How would I check this.

shp2pgsql applied no transformation, use your eyes to
check if the coordinates make sense as decimal degrees.

--strk;

> 
> 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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> > > >
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