[postgis-devel] PostGIS 3D - Points to be discussed

[Nicklas Avén]

Hallo Olivier and Hugo

First about solids. I am still confused. What tells if a closed
polyhedral surface is a surface or a solid? 


Next about planes.

When I wrote the distance code for 3D I was struggling some with how to
handle tha plane of a surface. If there is more than three 3D points
defining a plane it will in most cases not be coplanar caused by
precision errors. 

Then no matter how good the algorithm is for calculating the distances,
the answer will not be exact since the stored points defining the
surface is not coplanar. 

In the 3D distance calculations in PostGIS today some average plane is
calculated. But if it is a need for exact distance values I guess that
will be a problem.



I can only see two ways around that to really get exact calculations.

1) store some definition of the plane in the geometry
2) define 3 points in the geometry that defines the plane. The rest of
the points is regarded as 2D points.

In the second way of doing it the 2D points will not always be xy points
but xy, xz and yz points depending on the plane. 

Thanks

Nicklas


On Fri, 2013-02-01 at 07:46 +0100, Oliver Courtin wrote:
> 
> Le 31 janv. 2013 à 21:39, Nicklas Avén a écrit :
> 
> 
> Hi Nicklas,
> 
> 
> To complete Hugo previous answer,
> 
> > Interesting things. I cannot say I see at once what it it all means,
> > but
> > I have a question.
> > 
> > We were discussing this long time ago, but I don't think I have
> > understood yet :-)
> > 
> 
> 
> :)
> 
> > In 3D when you talk about a solid. How is that defined. As I
> > remember
> > you meant that it is a solid if it is closed. But I think that
> > sounds
> > strange. It sounds like a closed linestring have to be a polygon.
> > But it
> > dosn't. It can be just a border or just a "closed linestring". 
> > 
> > How do we flag that a closed surface is a solid and not just a
> > "skin".
> > 
> 
> 
> It is a bit strange because (for now) SFS 1.2 use the same type to
> represent
> both surfaces in 3D and also solids (since the surface is closed).
> 
> 
> Maybe/surely with coming SFS standard, solids could have distinct WKT 
> types. But in the meantime...
> 
> 
> > If discussing distances. The distance from the table to the wall
> > might
> > be 2 meters. But if the house is to be regarded as a solid it is 0.0
> > meters.
> > 
> > I'm I thinking wrong or do we have to expand the 3D type(s) with
> > some
> > solid type?
> > 
> 
> 
> Nothing to be done related to 3D type, right now (it's already there).
> But you're right if some 3D functions are not aware that surfaces 
> (Tin / PolyhedralSurface) could also represent solid, 
> well it's a missing feature...
> 
> 
> 
> 
> O.
> 
> 
> 
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