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

From: "Tim Dunn" <[email-masked]>
Date: Tue, 27 Sep 2005 08:21:58 -0600

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.

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.

-- 
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---

From: "James M. Polk" <[email-masked]>
Date: Tue, 27 Sep 2005 09:46:09 -0500

>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.

>understand it the conversion isn't exact and I'd hate a rounding error to

cheers,
James

                                 *******************
                 Truth is not to be argued... it is to be presented.

---

From: "Clive D.W. Feather" <[email-masked]>
Date: Thu, 29 Sep 2005 10:16:37 +0100

> How about an alternate presentation of the requirement, specifically:
[...]

Having read the discussion over the last couple of days, what I think is
needed is:

A:  There MUST be a way in the protocol for the caller's equipment to
    negotiate a choice of coordinate system with the PSAP.
Aa: Calling equipment MUST be able to convert its location to any of the
    coordinate systems it offers in the negotiation.
Ab: PSAP equipment MUST be able to convert the received location from any
    of the coordinate systems it offers in the negotiation to that
    required internally.
B:  Both calling equipment and PSAP MUST offer WGS-84 as part of the
    negotiation (even if not the first choice). [This could be implemented
    either by requiring it to be on any lists, or by making it the fallback
    if negotiation fails.]

Then, if the negotiation is the common one of "offer an ordered list, use
the first one on both side's lists":

    UK CPE offers:         OSGB, WGS-84
    other CPE offers:      XXX, YYY, WGS-84
    UK PSAP accepts:       OSGB, WGS-84
    other PSAP accepts:    ZZZ, WGS-84

(where XXX, YYY, ZZZ are other systems) and all works.