[postgis-devel] ST_Perimeter and ST_Perimeter3D what should Polyhedral Surface return?

[Olivier Courtin]

On Aug 13, 2010, at 7:43 PM, Nicklas Avén wrote:

Hi all,

> This about perimeter in 3D.
> When I thought about it it lead me to the question when a in  
> polyhedral
> surface becomes a polyhedron. Like a closed linestring, can be a
> linestring without turning to a polygon.
>
> The two questions connect in that, when the surface turns to a volum I
> guess there is no perimeter like there is no length of line when the
> linestring becomes a polygon.

You're right Nicklas, when the PolyhedralSurface (and now TIN)
are Closed, the Perimeter3D have to return 0

And as Regina said, the Perimeter/Perimeter3D should only
care about 'external' edge length.
In fact all the edges connected to only one face.

I create a ticket on this

> Olivier, I thought about it also because of the discussion the other  
> day
> about testing if a point is inside a closed polyhedral when doing
> distance-measures. If there is no way to show that it is a polyhedron
> there should be the distance to the surfaces even if the point is
> inside?
> I mean the distance from a point to a closed linestring is the  
> distance
> to the linestring even if the point is inside the closed line.


After our discussion, i really think that having only LWGEOM struct
for connected surfaces (PolyhedralSurface, TIN) is not enough.

We will spend a lot of time/energy in each spatial function to compute
again the topology beneath.

There's a need to have a point/edge/face model.
Need to explore this way...

--
Olivier