[postgis-users] Area and scale completely wrong

[Johan Wehtje]

Thanks Baren and strk - that was the problem, the file was in decimal
degrees.

Can these files be used in Postgis ? is it just a matter of finding another
SRID, or do I have to convert them sowehow?

Cheers
Johan Wehtje

On 11/14/05, Barend Köbben <kobben at itc.nl> wrote:
>
> In ArcGIS/ArcView the original projection of the data can be different
> from the projection of the View (or data frame in ArcGIS). EG. you data
> could be in decimal degrees, 0,0 being the convergence of the equator with
> the Greenwich meridian, while it could be projected to meters in UTM where
> 0,0 could be quite another place on earth. In ArcGIS you can see the
> projections of the data layer and the data frame by looking at the
> propoerties of these two things (right clicking them). In ArcView I think
> (don't have it here) you sholdl be able to see that data too of respectively
> the layer and the View. The easiest wai in PostGIS is just lloking at the
> numbers of the coordinates: if you
> select AsText(geom) from your table and see coordinates like 2.93883558758
> 10.227323263, you're looking at decimal degrees latitude-longitude, if you
> see coords like 2125353.32 12263535.33 it's meter coordinates in some
> cartesian system...
>
>
> ___________________________
>
> Barend Köbben
> International Institute for Geo-information
> Sciences and Earth Observation (ITC)
> PO Box 6, 7500AA Enschede (The Netherlands)
> ph: +31 (0)53 4874253; fax: +31 (0)53 4874335
>
>
>
>
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net
> > [mailto:postgis-users-bounces at postgis.refractions.net] On
> > Behalf Of Johan Wehtje
> > Sent: 14 November 2005 11:34
> > To: PostGIS Users Discussion
> > Subject: Re: [postgis-users] Area and scale completely wrong
> >
> > 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 <mailto: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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