Re: [atoca] Next milestone

"Carl Reed" <creed@opengeospatial.org> Tue, 25 September 2012 21:23 UTC

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Message-ID: <3DF5430D69FC4F6A936F45A1F6805E08@OfficeHP>
From: "Carl Reed" <creed@opengeospatial.org>
To: "Art Botterell" <acb@incident.com>, "Brian Rosen" <br@brianrosen.net>
References: <CABkgnnXJQ25CRw4wVGmvk3tBCKUZGNgei4KOVseFBYJfotxb=Q@mail.gmail.com><1597D6F5-A6A4-418C-AE5C-C4426992A645@brianrosen.net><67ADBEFC-B056-4064-B18C-02EB9CBC539E@incident.com><A8BDD10D-479A-4D72-B565-439DD6923F71@brianrosen.net><783AAAE3-344E-4C03-B3FB-E402F84D0699@incident.com><E7978A1A74C14CCF94702F4C4816DCAC@OfficeHP><C45316E8-46F1-42A0-85C2-B73EFAA0408B@incident.com><B9135A12-7D34-459C-8529-CE0F615FBC6A@brianrosen.net><5B654303-65A2-4BD8-AAC3-7A986FA2C2AE@incident.com><D935B591-6BB0-49BB-811A-2E6B3275B141@brianrosen.net> <D26D3E61-64E7-4B4D-8666-8B0DCABF1E19@incident.com>
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Date: Tue, 25 Sep 2012 15:23:05 -0600
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Subject: Re: [atoca] Next milestone
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A side note.

KML (formally Keyhole Markup Language) was originally defined as part of a 
contract Keyhole had with the EUSC. The EUSC suggested that keyhole look at 
GML 2.1.2, which they did. The underlying geometry model for KML and GML are 
the same. Both are based on ISO 19107 (as are many other geometry encoding 
patterns, such as GeoJSON and SQL-MM). Unfortunately, Keyhole decided to 
deviate in a few definitions, such as coordinate pairs are coded long-lat 
instead of lat-long (which is the geodetic tradition). They were trying to 
be consistent with some GIS technology back in the late 1990s - to bad. 
Should have followed the standards and not some vendor specific approach to 
defining polygons and coordinate strings.

Anyway, if one looks at the definition of KML polygon:

A Polygon is defined by an outer boundary and 0 or more inner boundaries. 
The boundaries, in turn, are defined by LinearRings.

And LinearRings is also defined - identical to GML LinearRing.

Apologies for deviating from the discussion.

Cheers

Carl

-----Original Message----- 
From: Art Botterell
Sent: Tuesday, September 25, 2012 2:36 PM
To: Brian Rosen
Cc: atoca@ietf.org
Subject: Re: [atoca] Next milestone

Please, Brian, look at the spec before generalizing about it.  All those 
questions are precisely answered.  The answers are different than those for 
either GML or KML, but no less detailed.


On Sep 25, 2012, at 1:33 PM, Brian Rosen wrote:

> How are the pairs separated?
> What whitespace is acceptable?
> Is whitespace allowed between the coordinates, before or after the comma?
>
> Stuff like that.
>
> Brian
> On Sep 25, 2012, at 4:15 PM, Art Botterell <acb@incident.com>; wrote:
>
>> Personally I'd have no objection to using a simple GML-based 
>> representation, although that would mean that the "wrapper" would have a 
>> different serialization of a geometry than the enclosed CAP message, 
>> which some might find confusing or inconsistent even though the two would 
>> be semantically equivalent.
>>
>> (And again, the underlying question is whether the world actually needs 
>> yet another "wrapper" for alerts, considering that we already have both 
>> SOAP and the OASIS EDXL-DE.)
>>
>> But I do have to question your assertion that the CAP format is either 
>> "ill-defined" or "subject to interpretation."  Have you actually read the 
>> CAP spec?  If so, what exactly do you mean?
>>
>> - Art
>>
>>
>> On Sep 25, 2012, at 1:04 PM, Brian Rosen wrote:
>>
>>> Right, but what do we do here?
>>>
>>> In general, in the IETF, we use GML.  CAP doesn't, and has an 
>>> ill-defined, subject to interpretation, representation, as you said 
>>> below.
>>>
>>> What should we do?  We need interoperability.  We could try to profile 
>>> it so we get interoperability, and describe how that would be converted 
>>> to GML I suppose.
>>>
>>> Brian
>>>
>>> On Sep 25, 2012, at 3:58 PM, Art Botterell <acb@incident.com>; wrote:
>>>
>>>> Dunno, Carl... certainly we were glad to have you there on the TC 
>>>> representing OGC... and yet somehow, despite all our best intentions, 
>>>> the coordinate order wound up reversed in CAP vs GML.  And then, of 
>>>> course, KML came into play, different yet again.  But 
>>>> standards-political history isn't the topic here.  The practical point 
>>>> is that there are a variety of ways out there to say what's ultimately 
>>>> the exact same thing.
>>>>
>>>> - Art
>>>>
>>>> On Sep 25, 2012, at 12:28 PM, Carl Reed wrote:
>>>>
>>>>> Art
>>>>>
>>>>> GML changed directions? Not sure what you mean. There is now an OASIS 
>>>>> GML profile for use in OASIS EM standards. Being referenced in HAVE, 
>>>>> xAL, and EDXL. Perhaps in the next version of CAP? :-)
>>>>>
>>>>> Cheers
>>>>>
>>>>> Carl
>>>>>
>>>>>
>>>>> -----Original Message----- From: Art Botterell
>>>>> Sent: Tuesday, September 25, 2012 12:51 PM
>>>>> To: Brian Rosen
>>>>> Cc: atoca@ietf.org
>>>>> Subject: Re: [atoca] Next milestone
>>>>>
>>>>> Funny thing, that... we tried to make the CAP geometries align with 
>>>>> GML... even has a representative from OGC (Carl Reed) on the OASIS 
>>>>> committee when we were' specifying it... but then GML changed 
>>>>> directions and the CAP spec wound up different.  Personally I'd 
>>>>> suggest using the CAP syntax, for consistency, as that can be switched 
>>>>> into GML, KML or whatever easily and precisely an application 
>>>>> requires.
>>>>>
>>>>> Yes, ultimately any geocode is really just a shorthand for some 
>>>>> polygon. Different jurisdictions around the world will have different 
>>>>> geocoding schemes, and it's hard to assume that every receiving device 
>>>>> will be familiar with them all... but geometry is universal... or at 
>>>>> least planet-wide.
>>>>>
>>>>> - Art
>>>>>
>>>>>
>>>>> On Sep 25, 2012, at 11:25 AM, Brian Rosen wrote:
>>>>>
>>>>>> I guess that's my point.  Without more spec, we have interoperability 
>>>>>> issues.  When you have limited CAP deployment, these can be dealt 
>>>>>> with by prior arrangement.  We probably can't do that.  I would 
>>>>>> prefer that a polygon be encoded with GML, which is precise, 
>>>>>> interoperable and an accepted global standard.  That can be wrapped 
>>>>>> in a PIDF (an IETF standard) or not, but we need something very 
>>>>>> interoperable.
>>>>>>
>>>>>> Forcing polygons may work in a forward looking sense.   I'm probably 
>>>>>> convincible that a jurisdictional areas aren't needed, but so many of 
>>>>>> the alert sources I'm interested in supporting are based on 
>>>>>> jurisdictions.  I guess just telling them they need to come up with a 
>>>>>> relatively simple polygon is okay.  Since increasingly, device 
>>>>>> location is GPS (or equivalent) based, we probably need geo always.
>>>>>>
>>>>>> Brian
>>>>>>
>>>>>>
>>>>>> On Sep 25, 2012, at 2:14 PM, Art Botterell <acb@incident.com>; wrote:
>>>>>>
>>>>>>> On Sep 25, 2012, at 10:30 AM, Brian Rosen wrote:
>>>>>>>> For example, how do you target "Allegheny County, PA, US"?  A text 
>>>>>>>> line? With what syntax?  A polygon?
>>>>>>>
>>>>>>>
>>>>>>> In CAP the recommended presentation would be a polygon, a line of 
>>>>>>> text that might well read "Allegheny County, PA, US" and optionally 
>>>>>>> a <geocode> value, which in the US would typically be a FIPS-based 
>>>>>>> code conforming to the old Emergency Alert System standard (in this 
>>>>>>> case, "042003").
>>>>>>>
>>>>>>> There are a few subtleties here.  First, most hazard footprints 
>>>>>>> don't align very well with political boundaries, so it's actually 
>>>>>>> considered best practice NOT to address an alert to everyone in a 
>>>>>>> political jurisdiction if better resolution is available, which 
>>>>>>> increasingly often is the case.  (This is probably the biggest 
>>>>>>> challenges in the transition from the legacy EAS and Weather Radio 
>>>>>>> systems in the States and the newer CAP-based IPAWS framework.)
>>>>>>>
>>>>>>> Second, but related to the first... there is at present no strict 
>>>>>>> limit on the number of vertices in a polygon in CAP.  Generally this 
>>>>>>> hasn't been a problem, as both manual and model-derived (e.g., from 
>>>>>>> a hazardous materials "plume model" software) polygons tend to be 
>>>>>>> relatively simple. However, if one uses a political jurisdiction as 
>>>>>>> the target area and if, as often is the case, that jurisdiction is 
>>>>>>> defined at least in part by a waterway or water body, then at least 
>>>>>>> part of the "true" polygon will be fractal in nature and the number 
>>>>>>> of vertices used to represent it becomes an implementation decision.
>>>>>>>
>>>>>>> In such cases conversion from GIS geometries developed for other 
>>>>>>> purposes can, at least in theory, produce very lengthy polygon 
>>>>>>> description strings.  Some degree of polygon simplification, 
>>>>>>> manually or by means of a "convex hull" calculation, may be helpful 
>>>>>>> in such situations; fortunately those can be calculated ahead of 
>>>>>>> time.  A common guideline and a requirement of the IPAWS profile in 
>>>>>>> the U.S. is that a polygon be limited to 100 vertices.  Again, this 
>>>>>>> is largely a theoretical concern, and I'm not aware of any case 
>>>>>>> where this has actually been a problem.
>>>>>>>
>>>>>>> And third, of course, there's quite a bit of variety in how polygons 
>>>>>>> get represented as strings.  Latitude first and then longitude, or 
>>>>>>> the reverse "x,y" order?  Commas between the coordinates and spaces 
>>>>>>> between the pairs, or vice versa, or some other delimiters?  All 
>>>>>>> such representations are ultimately equivalent (at least until we 
>>>>>>> drill down to the level of whether we're using the increasingly 
>>>>>>> common GPS-standard WGS84 datum or some model of the precise shape 
>>>>>>> of the Earth.)  But it's important to be clear which format is being 
>>>>>>> used and to remember of convert as necessary, e.g., between CAP and 
>>>>>>> KML.
>>>>>>>
>>>>>>> The CAP spec deprecates the use of geocodes (arbitrary string 
>>>>>>> designators) without also providing the corresponding geometry 
>>>>>>> (polygon), since to do otherwise is to assume that the receiving 
>>>>>>> device knows every possible geocoding scheme it might ever 
>>>>>>> encounter, which in an open global system is probably infeasible. 
>>>>>>> Indeed, ideally we might have done without geocodes entirely, but 
>>>>>>> back-compatibility with legacy systems (and the comfort of some 
>>>>>>> non-GIS-savvy programmers who understood string-matching but were 
>>>>>>> uncomfortable with things like point-in-polygon algorithms) dictated 
>>>>>>> their inclusion.
>>>>>>>
>>>>>>> And the text areaDescription field is defined merely as "human 
>>>>>>> readable" so it can be as concise or as extensive as circumstances 
>>>>>>> dictate. However, where multi-lingual alerting is required it tends 
>>>>>>> to be kept short to facilitate translation.
>>>>>>>
>>>>>>> -  Art
>>>>>>> _______________________________________________
>>>>>>> atoca mailing list
>>>>>>> atoca@ietf.org
>>>>>>> https://www.ietf.org/mailman/listinfo/atoca
>>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> atoca mailing list
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>>>>
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>>
>

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