[Gvsig_english] Fwd: PostGis + 3D extension
Benjamin Ducke
benjamin.ducke at oxfordarch.co.uk
Wed Jan 19 16:47:24 CET 2011
Your questions are not naive at all.
They touch a problem that many GIS users increasingly face.
The problem is that the roots of GIS are in digital cartography,
which is a 2D discipline. So the GIS datamodels were all originally
designed for 2D, including the vector model. Raster elevation
layers served the purpose as simple pseudo-3D models ("2.5 D").
As time went by, more and more people from "3D fields", such as
geology created a demand for real 3D data management, so capabilities
to load, view, and (to a limited degree) add 3D data was introduced.
And some GIS (such as GRASS GIS) also learned a limited amount of
3D analysis.
But many fundamental GIS aspects, such as topology, remain firmly
in the 2D domain, and it will be a long time before that changes,
because 3D topological relationships are a whole lot more complex
than 2D ones. Currently, 3D remains an "add-on" to GIS, that works
more or less well (and then there is 4D, but let's not even get
into that).
Polygons versus more complex 3D shapes are a good example. By
definition a polygon is a planar construct. Anything more complex than
that must be sub-divided into simpler polygons (usually 3D triangles),
so the GIS vector model can handle them. E.g. a TIN is just a collection
of simple triangles. And any other shape can also be decomposed into
triangular "faces".
Things are a little more simple for 3D volumetric rasters (voxels),
since they have an easy structure. The only open source GIS that
can currently handle voxels is GRASS GIS.
Through SEXTANTE and the GRASS GIS interface, we have access to the
GRASS voxels tools in theory, but the GUI interface must be extended first
to allow voxel inputs and outputs.
-- And I don't think the gvSIG 3D extension handles voxels yet (?)
Cheers,
Ben
----- Original Message -----
> Yes, you are right.
>
> What it is about is how to handle "dirty" polygons that is invalid.
> How to find that they are invalid without putting to much effort in
> it.
>
> The isValid function is a GEOS-function and only handles x z
> coordinates as far as I know, so a part of extending PostGIS
> capabilities in 3D is
> taking care of validation. Those functions is not in place yet (I
> think, I have if something new has been committed there by the guy
> implementing TIN and Polyhedralsurface)
>
> Do you have a good way of validating that the polygon is coplanar in
> gvSIG, I guess you have to handle invalid data too. How do you
> validate the data?
>
> Or same question from another angle. How do you define the plane in 3D
> extension?
>
> Or what happens if the extension gets the polygon I had as example?
> I will try when I get it up running :-)
>
> I am a novice in 3D so sorry for maybe naive questions.
>
>
> Regards
> Nicklas
>
>
>
>
>
> On Wed, 2011-01-19 at 16:03 +0100, Jordi Torres wrote:
> > I missed the link... :S
> >
> > [1]http://postgis.refractions.net/docs/ch04.html
> >
> >
> > 2011/1/19 Jordi Torres <jtorresfabra at gmail.com>
> > Hi Nicklas,
> >
> > What I meant is that a polygon is a two-dimensional shape,
> > enclosing an area with its vertices that is in the same
> > plane, coplanar. If you don't define your polygons this way
> > it's impossible to know how to tesellate and render it. I
> > don't know about the specification of polygons in postgis,
> > but maybe
> > it's up to the user to define its vertices correctly. I
> > found this[1] in the postgis doc, see point 4.3.5, maybe it
> > helps.
> >
> > Greetz.
> >
> > [1]
> >
> >
> > 2011/1/19 Nicklas Avén <nicklas.aven at jordogskog.no>
> > sorry about off list posting
> >
> >
> >
> >
> > On Wed, 2011-01-19 at 14:35 +0100, Jordi Torres
> > wrote:
> > > Hi Nicklas,
> > >
> > > Please, let' s take this conversation "on-list".
> > > The
> > attacments did
> > > not pass the list control, that's the reason why I
> > wrote to you
> > > directly. About generating 3D datasets it's simple
> > > if you have
> > the data in .shp
> > > (using shp2pgsql). That's the way I get my data.
> > > In the other hand, a polygon cannot be in more
> > > than
> > one plane in 3D,
> > > if this is the case, this structure must be
> > tessellated into triangles
> > > or at least polygons in the same plane.
> >
> >
> > Well, what do you mean by cannot be in more than one
> > plan?
> >
> > I understand that it cot can when it comes to
> > rendering and so on.
> > And it cannot when it comes to valid polygons.
> >
> > But you can express a polygon in wkt that has
> > vertexpoints in more than
> > one plan.
> >
> > for example:
> > POLYGON((0 0 0,0 5 0, 0 5 5, 10 5 5, 10 5 0, 10 0 0,
> > 0 0 0))
> >
> > In many examples the result is undefined and
> > impossible to decide
> > without more information and that's why I have to
> > deal with it in
> > distance calculations and send an error msg instead
> > of trying to
> > calculate the distance.
> >
> > Or am I missing something?
> >
> > Regards
> > Nicklas
> >
> >
> >
> >
> >
> > > (the same as openGL do when rendering concave
> > polygons). May be the
> > > tesselator included in the 3D extension eats this
> > kind of structure,
> > > but I don't trust.
> > >
> > > About PostGis I am a noob or less than a noob. I
> > only know the basics
> > > to set a database and do a sql query. And I
> > > learned
> > it yesterday :). I
> > > am a 3D developer, and postgis is a tangential
> > theme for me. So for
> > > the questions about postGIS ask to the gurus in
> > > the
> > list... ;)
> > >
> > > Cheers.
> > >
> > >
> > > 2011/1/19 Nicklas Avén
> > > <nicklas.aven at jordogskog.no>
> > > Yes, that would be nice :-)
> > >
> > > It is a little bit difficult to create 3D
> > datasets I think, at
> > > least if
> > > the polygons contains more than 3 points
> > (counting the first
> > > and last as
> > > 1) because they have to be in the same
> > > plan.
> > That is the part
> > > in the 3D
> > > distance code that I am most worried
> > > about.
> > Now there is no
> > > real test if
> > > people sends polygons not on one single
> > > plan
> > to the function,
> > > because such a test is quite expensive to
> > > calculate
> > if the polygon has
> > > many points. Also there have to be some
> > tolerance.
> > >
> > > Have you tried the PostGIS trunk lately?
> > >
> > > It will be interesting to test and
> > > visualize st_3d_shortestline through
> > > gvSIG.
> > >
> > > Cheers
> > > Nicklas
> > >
> > >
> > > On Wed, 2011-01-19 at 13:55 +0100, Jordi
> > Torres wrote:
> > > > Hi Nick,
> > > >
> > > > Is a lot of triangular separate
> > > > polygons,
> > if you want the
> > > archive to
> > > > do testing I can send it to you.
> > > >
> > > > Cheers.
> > > >
> > > > 2011/1/19 Nicklas Avén
> > <nicklas.aven at jordogskog.no>
> > > > Thanks a lot
> > > >
> > > > I will try it as soon as
> > > > possible.
> > When I have this
> > > woking
> > > > there will be
> > > > a blog post about this extension
> > and comming 3d
> > > distance
> > > > functions in
> > > > postgis.
> > > >
> > > > Is TIN supported? or is it a lot
> > of triangular
> > > separate
> > > > polygons in the
> > > > example picture?
> > > >
> > > > Great work!
> > > > Thanks
> > > >
> > > > /Nicklas
> > > >
> > > >
> > > >
> > > > On Wed, 2011-01-19 at 13:47
> > > > +0100,
> > Jordi Torres
> > > wrote:
> > > > >
> > > > >
> > > > > ---------- Forwarded message
> > ----------
> > > > > From: Jordi Torres
> > <jtorresfabra at gmail.com>
> > > > > Date: 2011/1/19
> > > > > Subject: Re: [Gvsig_english]
> > PostGis + 3D
> > > extension
> > > > > To: Users and Developers
> > > > > mailing
> > list
> > > > >
> > <gvsig_internacional at listserv.gva.es>
> > > > >
> > > > >
> > > > > Hi Nicklas,
> > > > >
> > > > > Here you have attached a zip
> > file. Substitute the
> > > jars in
> > > > the way Fran
> > > > > told to you.
> > > > >
> > > > > And congrats to jaspa teams!!
> > Good work!
> > > > >
> > > > > Cheers.
> > > > >
> > > > > 2011/1/19 Francisco José
> > Peñarrubia
> > > <fpenarru at gmail.com>
> > > > >
> > > > >
> > > > > Hi.
> > > > >
> > > > > I just want to remark
> > and say thanks to
> > > JASPA team,
> > > > who did
> > > > > the hard work of this
> > patch in gvSIG
> > > Sprint. I hope
> > > > we can use
> > > > > it in 1.11 release.
> > > > >
> > > > >
> > http://jaspa.forge.osor.eu/
> > > > >
> > > > > Best regards.
> > > > >
> > > > > Jordi, please, can you
> > send the latest
> > > jars to
> > > > Niklas?. I will
> > > > > create a patch and
> > upload it to the
> > > repository.
> > > > >
> > > > > El 19/01/2011 13:30,
> > Jordi Torres
> > > escribió:
> > > > > >
> > > > > > Hi Nick and Fran,
> > > > > >
> > > > > > I have started a new
> > thread in the list,
> > > this way
> > > > we won't
> > > > > > merge topics.
> > > > > >
> > > > > > Fran has fixed the
> > issues and now we are
> > > able to
> > > > see 3D
> > > > > > geometries using
> > postigs (attached is a
> > > screenshot
> > > > of a 3D
> > > > > > polygons layer). By
> > the way, polyhedral
> > > is not
> > > > supported by
> > > > > > gvSIG postGis driver
> > (even I don't now
> > > if it is
> > > > OGC
> > > > > > compliant), so you
> > have to decompose it
> > > in
> > > > polygons.
> > > > > >
> > > > > > Cheers!
> > > > > >
> > > > > > -- Jordi Torres
> > > > > > Fabra
> > > > > >
> > > > > > gvSIG 3D blog
> > > > > >
> > http://gvsig3d.blogspot.com
> > > > > > Instituto de
> > Automática e Informática
> > > Industrial
> > > > > > http://www.ai2.upv.es
> > > > > >
> > > > > >
> > > > > >
> > >
> > _______________________________________________
> > > > > > Gvsig_internacional
> > mailing list
> > > > > >
> > Gvsig_internacional at listserv.gva.es
> > > > > >
> > > >
> > >
> > http://listserv.gva.es/cgi-bin/mailman/listinfo/gvsig_internacional
> > > > >
> > > > > -- Fran Peñarrubia
> > > > > Scolab www.scolab.es
> > > > >
> > > > > Asociación gvSIG
> > > > > www.gvsig.com
> > > > >
> > > > >
> > >
> > _______________________________________________
> > > > > Gvsig_internacional
> > mailing list
> > > > >
> > Gvsig_internacional at listserv.gva.es
> > > > >
> > > >
> > >
> > http://listserv.gva.es/cgi-bin/mailman/listinfo/gvsig_internacional
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > -- Jordi Torres Fabra
> > > > >
> > > > > gvSIG 3D blog
> > > > > http://gvsig3d.blogspot.com
> > > > > Instituto de Automática e
> > Informática Industrial
> > > > > http://www.ai2.upv.es
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > -- Jordi Torres Fabra
> > > > >
> > > > > gvSIG 3D blog
> > > > > http://gvsig3d.blogspot.com
> > > > > Instituto de Automática e
> > Informática Industrial
> > > > > http://www.ai2.upv.es
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > -- Jordi Torres Fabra
> > > >
> > > > gvSIG 3D blog
> > > > http://gvsig3d.blogspot.com Instituto de
> > > > Automática e Informática
> > Industrial
> > > > http://www.ai2.upv.es
> > >
> > >
> > >
> > >
> > >
> > >
> > > -- Jordi Torres Fabra
> > >
> > > gvSIG 3D blog
> > > http://gvsig3d.blogspot.com Instituto de
> > > Automática e Informática Industrial
> > > http://www.ai2.upv.es
> >
> >
> >
> >
> >
> >
> >
> > --
> >
> > Jordi Torres Fabra
> >
> > gvSIG 3D blog
> > http://gvsig3d.blogspot.com Instituto de Automática e
> > Informática Industrial
> > http://www.ai2.upv.es
> >
> >
> >
> >
> > -- Jordi Torres Fabra
> >
> > gvSIG 3D blog
> > http://gvsig3d.blogspot.com Instituto de Automática e Informática
> > Industrial
> > http://www.ai2.upv.es
> > _______________________________________________ Gvsig_internacional
> > mailing list
> > Gvsig_internacional at listserv.gva.es
> > http://listserv.gva.es/cgi-bin/mailman/listinfo/gvsig_internacional
>
>
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