Subject: [Geopriv] issues and adoption of draft-thomson-geopriv-geo-shape

From: Andrew Newton <[email-masked]>
Date: Tue, 09 May 2006 10:18:58 -0400

All,

Martin Thomson's document titled "Geodetic Shapes for the Representation 
of Uncertainty in PIDF-LO" (draft-ietf-geopriv-geo-shape-01.txt) has 
received positive review and support for adoption as a working group item.

As co-chair of the working group, I have two questions for participants 
who have not voiced their opinion of this document:

1) Are there any unresolved issues with this document?  And if so, is it 
your opinion that these issues cannot be resolved?

2) Are there any reasons (technical or non-technical) for not adopting 
this document as a GEOPRIV work item?

I would appreciate this feedback no later than 16 May 2006.

-andy

_______________________________________________
Geopriv mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/geopriv

---

From: "Carl Reed OGC Account" <[email-masked]>
Date: Tue, 9 May 2006 08:43:02 -0600

I would like to add that three OGC member organization representatives - who 
also happen to be GML experts and authors of the GML standard - have given a 
very positive thumbs up to the accuracy of the GML content in this document. 
Further, they believe that a number of the geometry types defined in this 
document that GML does not support should be considered as change requests 
for enhancement to GML.

Regards

Carl

---

From: Cullen Jennings <[email-masked]>
Date: Tue, 9 May 2006 10:03:26 -0700

I am going to bring up some issues which I imagine have been  
discussed to death in the WG but are some random thoughts that came  
up when I first read the document. I imagine there are good answers  
to all of them but I did not find them in the draft so help me get up  
to speed on all this stuff. PS - I definitely agree we need a good  
document specifying this type of stuff and I'm all for re-use of GML.

Let's say I had the polygon specified in WGC 84 lat, Lon,  alt,  and  
the vertex's are

120, 45, 0
120, 46, 0
120, 47, 0
115, 46, 0

Is that a co-planar polygon? The see how the draft says it is not  
then tries to create wiggle room around claiming it is. Pretending  
that a spherical coordinate system is actually a cartesian coordinate  
system is fought with disaster - particularly around the poles.

What is the shape of the following polygon

90,0,0
-90,0,0
0,180,0

The definition of the arc band used here, when specified in a  
spherical coordinate system results in a shape that is remarkable  
complex to decide if a line intercepts it or not.

Imagine I am trying to specify the polygon that defines the southern  
hemisphere - the proposed normal algorithm looks like it will have  
some issues. I will be somewhat suspicious off any solution in a  
spherical coordinate system that does not require polygons to be  
specified in CW (or CCW) direction.

It was unclear to me how the angles for the arc bands where specified.

It's not clear to me what shapes you need and what operations you  
need on them. Off the top of my head it seams like you need the  
following.

0) something that works in all cases

1) Specify a point in spherical coordinate system
2) Find great circle distance between two points

3) Specify a spherical polygon of constant altitude
- by this I mean great edges are great circle arcs between vertex
- all vertex have same altitude
- surface is a subset of a spherical shell
- correctly specifies and deal with singularities issue and  
degenerate polygons
(I'm sure there is a better way of saying all this but you get the idea)

4) Specify a spherical prism made by the space bounded by a polygon  
at one altitude and an polygon with vertexes with identical lat,lon  
but different altitude.

5) decide if point is strictly outside of spherical prism.

If the Uncertainty stuff is needed, then it seems you would also need

6) decide the great circle distance from a point to a spherical prism.
- I have no idea how hard it is to compute this but I would want to  
know before we decided to do it

If uncertainly needs to be expressed by something that does not  
locally approximate a circle, well, life is going to get way more  
complex. I can imagine that many of the location approaches would  
give error regions that looked locally like an ellipse or the arc  
band. If we really need this, we need to seriously consider what  
operation we need and the complexity of intersecting the 3d spherical  
version of an arc band or ellipse with a spherical prism. If it is  
possible to leave this out of the first version of the specification,  
well life will be easier.

I have not seen a need to decide if some line intersects a prism  but  
we should think about if we have a use case for that or not.

So, forgive me as I still getting up to speed on this. It was not  
clear to me exactly what we needed from a geometry system. That made  
it harder to decide what parts were missing from this draft and what  
parts that are in could be removed.

Cullen <with my individual contributor hat on>

On May 9, 2006, at 7:18 AM, Andrew Newton wrote:

> All,
>
> Martin Thomson's document titled "Geodetic Shapes for the  
> Representation of Uncertainty in PIDF-LO" (draft-ietf-geopriv-geo- 
> shape-01.txt) has received positive review and support for adoption  
> as a working group item.
>
> As co-chair of the working group, I have two questions for  
> participants who have not voiced their opinion of this document:
>
> 1) Are there any unresolved issues with this document?  And if so,  
> is it your opinion that these issues cannot be resolved?
>
> 2) Are there any reasons (technical or non-technical) for not  
> adopting this document as a GEOPRIV work item?
>