Subject: RE: [Ecrit] Comment on ecrit-requirements-00 item L10

From: "Tim Dunn" <[email-masked]>
Date: Tue, 27 Sep 2005 09:11:04 -0600

I think, yes, we prefer WGS-84 (I read that as the crux of that original
requirement). But if you're using a coordinate system other than WGS-84, you
should have a facility that allows you to convert from that system to
WGS-84.

By the way I said I preferred must in the second requirement but was OK with
should (because I thought there would be consternation with requiring
coordinate transformation).

So to re-reword to a much more generic set of requirements that:

A:  The preferred coordinate reference system for emergency calls is WGS-84.
B:  Systems should have an ability to perform coordinate transformation
between differing coordinate reference systems.

Tim

-----Original Message-----
From: James M. Polk [mailto:[email-masked]] 
Sent: Tuesday, September 27, 2005 8:46 AM
To: Tim Dunn; 'Clive D.W. Feather'; 'ECRIT'
Subject: RE: [Ecrit] Comment on ecrit-requirements-00 item L10

At 08:21 AM 9/27/2005 -0600, Tim Dunn wrote:
>Clive-
>
>How about an alternate presentation of the requirement, specifically:
>
>A:  The preferred coordinate reference system for emergency calls is
WGS-84.
>B:  Systems using coordinate reference systems other than WGS-84
>(should/must) have the ability to perform coordinate transformation from
the
>alternate system to WGS-84 and vice-versa.

Tim

so what you're saying is:

         "we prefer you use WGS84, and if you don't, you (should/must)
           convert to/from it"

..this seems like a MUST to me dressed up to sound more palatable by those 
that don't use it.

I agree with Clive - we cannot have the words "preferred" and "MUST" in the 
same sentence anywhere.

>I feel the best classifier is MUST but I'm OK with SHOULD.
>
>Tim Dunn
>Timothy N. Dunn Consulting
>
>
>-----Original Message-----
>From: [email-masked] [mailto:[email-masked]] On Behalf Of
>Clive D.W. Feather
>Sent: Tuesday, September 27, 2005 2:07 AM
>To: ECRIT
>Subject: [Ecrit] Comment on ecrit-requirements-00 item L10
>
>Item L10 says that "The preferred coordinate reference system for emergency
>calls MUST be WGS-84".
>
>Firstly, I'm not sure how "MUST" interacts with "preferred".
>
>Secondly, in the UK the preferred coordinate system for emergency services
>is OSGB National Grid. Emergency services won't easily know what to do with
>a lat-long pair. Even if there is software at each end of the call to do
>the conversion, it feels the wrong thing to do; furthermore, as I
>understand it the conversion isn't exact and I'd hate a rounding error to
>creep in.
>
>--
>Clive D.W. Feather  | Work:  <[email-masked]>   | Tel:    +44 20 8495 6138
>Internet Expert     | Home:  <[email-masked]>  | Fax:    +44 870 051 9937
>Demon Internet      | WWW: http://www.davros.org | Mobile: +44 7973 377646
>Thus plc            |                            |
>
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cheers,
James

                                 *******************
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---

From: "James M. Polk" <[email-masked]>
Date: Tue, 27 Sep 2005 11:51:20 -0500

At 09:11 AM 9/27/2005 -0600, Tim Dunn wrote:
>between differing coordinate reference systems....

...to WGS84.

then I agree with this wording

cheers,
James

                                 *******************

---

From: "Tom-PT Taylor" <[email-masked]>
Date: Tue, 27 Sep 2005 14:20:23 -0400

I don't see why a UK system (not sure what "system" refers to, BTW) 
would want to convert to WGS-84.  I would think it would be the other 
way around.

---

From: Henning Schulzrinne <[email-masked]>
Date: Tue, 27 Sep 2005 16:11:28 -0400

Given that a phone made in Japan, say, has to work when the owner visits 
London, using a local mapping server, support for a common datum seems 
pretty much required. Otherwise, we have to have a discovery mechanism 
for mapping servers based on the datum they support - I don't think we'd 
want that. (Our core assumption is that there may not be a service 
provider, so the Japanese tourist in London may not have a "home" 
provider to go ask, either.)

---

From: "Tom-PT Taylor" <[email-masked]>
Date: Tue, 27 Sep 2005 16:27:29 -0400

We have two requirements:
(1) create a workable mapping system;
(2) provide location to the PSAP and beyond in a form they can use.

These two requirements seem to conflict if PSAPs and/or emergency 
services use something other than WGS-84.  The key issues are:
- where is the best place to do the conversion;
- do we have any hope of specifying the coordinate system that must be 
used when creating a geolocation?

>> would want to convert to WGS-84.  I would think it would be the other 
...

---

From: Henning Schulzrinne <[email-masked]>
Date: Tue, 27 Sep 2005 16:41:28 -0400

How much effort is datum conversion in practice?

Given that the number of mapping servers and PSAPs is orders of 
magnitude lower than the number of users and that those servers and 
PSAPs are operated by people who know something about datums (as opposed 
to most client developers...), I think putting the conversion burden on 
mapping servers and PSAPs is the better decision, unless there's no way 
to convert these data representations with reliability at all.

Tom-PT Taylor wrote:
>> Given that a phone made in Japan, say, has to work when the owner 
>> visits London, using a local mapping server, support for a common 
>> datum seems pretty much required. Otherwise, we have to have a 
>> discovery mechanism for mapping servers based on the datum they 
>> support - I don't think we'd want that. (Our core assumption is that 
>> there may not be a service provider, so the Japanese tourist in London 
>> may not have a "home" provider to go ask, either.)
>>
>> Tom-PT Taylor wrote:
>>