Darrell -
I respectfully both agree and disagree. Years ago a few of us in the TC
argued for CAP being able to support coordinate reference systems other than
WGS84 2d. At that time, we made little headway.
I work standards in many venues. Even the IETF community has recognized the
need to allow alternative CRSs than just WGS84. This is why, for example, the
location extension to DHCP provides for alternative CRSs to be used.
There is no issue with WGS84 being the default CRS for a CAP message. My
suggestion was to allow for the ability to express other CRS’s than WGS84. I was
told that this would add complexity and that many systems that would create or
ingest a CAP message would not “understand” any other CRS than WGS84. This
assumption I disagree with. Allowing alternative CRS definitions in the CAP
specification does not add complexity, increases flexibility, allows for
national profiles that adhere to national mandates as to which geoid to use,
protects for the future, as well as other advantages. And, most geotechnologies
can deal with CRS transforms – other than perhaps some of the very limited geo
capabilities in social media location APIs – such as from Twitter.
A couple of years ago, the EM TC agreed that at some point CAP 2.0 would be
developed. The 2.0 version of CAP would be enhanced to support the new OASIS GML
Simple Features profile that is now used in EDXL. An interesting aspect of such
a migration is that GML provides for a simple mechanism for expressing one or
more coordinate references systems.
I agree that we do not want one-off profiles for each case in which
national law or other best practice requires the use of a CRS other than WGS-84.
We in the OGC community have encountered this issue in India, China, South
Africa, and a few other countries. We have actually had to allow the Chinese to
modify our standards to use China’s mandated national CRS in schema examples in
order for them to become Chinese national standards.
So, as you point out, there may be an alternative solution in Australia but
this does not solve the broader global issue of CAP and the use of a CRS other
than WGS-84.
Regards
Carl Reed, PhD
CTO
Open Geospatial Consortium
Making location count!
From: Darrell O'Donnell
Sent: Monday, November 21, 2011 6:39 AM
To: Trott, Gregory
Cc: Elysa Jones ;
;
Subject: Re: [emergency] Question regarding use of WGS84 reference
system in CAP standard [SEC=UNCLASSIFIED]
Greg et al.
I will throw out a cautionary note about changing anything without making
it an explicit "different from CAP" addition. Though there is value in
supporting a different coordinate system, most groups have agreed that CAP is
strong because it uses a single, unambiguous, and widely used coordinate system.
The broad use of WGS84 has really opened up geospatial capabilities ranging from
web mapping, GPS, through to CAP.
The GIS and survey-world will argue differently and they have valid reasons
– they need to be hyper-accurate.
For alerting purposes we need to focus on using a widely known position
format and the minor inaccuracies here are not worth breaking this concept. Many
groups have argued for better positioning and different datums, but the CAP
community has consistently looked at its mission and realized that WGS84 meets
the needs of the broad community. In the case of GDA84, this would be consistent
with guidance issued in Australia, specifically by the Intergovernmental
Committee on Surveying and Mapping (ICSM). Here's a link (http://www.icsm.gov.au/gda/wgs84fact.pdf)
to an ICSM guidance site that uses the phrase "for most practical purposes GDA94
and WGS84 coordinates can be considered the same and no transformation is
required." There is a caveat to that statement in the document, that basically
states that if you need to be hyper-accurate, GDA94 needs to be explicitly
considered (see document for full detail). I will point out that for the
purposes of alerting, this kind of accuracy is meaningless. Alerting doesn't
operate at that level of accuracy.
I am happy to see that the ICSM has made this kind of guidance. As
Australia is leading an effort to tailor CAP to their needs, they are correctly
asking questions about the WGS84/GDA94 difference. From the ICSM I see a strong
argument in "living with" WGS84. Adding a new coordinate system to CAP will
cause widespread angst and complexity in the CAP alerting world – I'd hate to
see Australia have to ask each vendor to tailor their system to support GDA94,
as the costs are not worth the extra accuracy. Though the issue looks so small
at the XML level we need to be very careful about going in this direction – it
is a slippery slope.
For the GDA94 purists (I've been there - I get it!), I will throw out the
following idea. Keeping pure on the CAP side and allowing for the CAP-AU profile
to augment would potentially be good. A <parameter> or <geocode>
valueName/value pair could be used to support the use of GDA94 IN ADDITION to
the normative WGS84 values. That will allow the geo/survey savvy folk to use the
GDA94 values without breaking the systems that need WGS84. Here is an example of
the CAP Canadian Profile (www.cap–cp.ca) approach that uses a point.
<parameter>
<valueName>layer:CAPAN:eventLocation:point</valueName>
<value>60.52459336850855,-117.66350189992689</value>
</parameter>
The CAP-AU could make a similar reference (I'll make one up here):
<parameter>
<valueName>layer:CAP-AU:GDA94polygon</valueName>
<value>65.05568945905345,-122.5853768999256
63.76525349370743,-115.99358002492735 60.61097108402268,-113.62053314992798
57.542073550110594,-114.58733002492772 58.335165816909,-121.3549081499259
65.05568945905345,-122.5853768999256</value>
</parameter>
Hope this helps.
Cheers,
--
Darrell O'Donnell,
P.Eng.
President/Principal
Consultant
Continuum Loop
Inc.
+1.613.866.8904
From: "Trott, Gregory" <>
Date: Mon, 21 Nov 2011 16:49:13 +1100
To: ""
<>,
""
<>
Subject: [emergency] Question regarding use of
WGS84 reference system in CAP standard [SEC=UNCLASSIFIED]
UNCLASSIFIED
Elysa Jones asked me to
forward this email to the EM-TC and CAP PROFILES SC to seek your views on how
OASIS should handle an issue that has arisen in the development of the
Australian CAP Profile document with regard to the WGS84 coordinate
system.
The Normative Reference
list in the OASIS CAPv1.2 standard includes the WGS84 as the datum source for
geographic coordinates to be populated in CAP messages. The use of WGS84
is no longer appropriate for the Australian environment because the Geocentric Datum of Australia 1994 (GDA94) is now the
approved geographical coordinate system used in Australia. The GDA is a part of
the global coordinate reference frame and is directly compatible with the Global
Navigation Satellite Systems (GNSS), which is the generic term used to describe
the US Global Positioning System (GPS). Detailed information about
GDA94 can be obtained from Geoscience Australia at: http://www.ga.gov.au/earth-monitoring/geodesy/geodetic-datums/GDA.html
The question we need your
response to is: How should the different geocentric datums be managed within the
Australian CAP Profile document?
Two options that I feel
might be appropriate methods to manage these different datums in the Australian
CAP Profile are:
Option 1 – insert GDA94
into the Normative Reference list in the CAP-AU Profile document to replace the
existing WGS84 reference.
-
If any situation arose where a non-Australian CAP message was received that
provided geo location coordinates from a geo standard that is different to the
GDA94 (eg using the WGS84 or some other standard) then the organisation in
Australia who received the CAP message would not likely detect a problem with
the coordinates caused by use of different datums, until they had to act on the
coordinates and accurately find where the hazard was located (perhaps to focus a
satellite on the problem area or direct a rescue / recovery team to the hazard
location).
-
A Note should be added to the AREA element to highlight there is a different
datum system used in Australia and explain the potential for location errors not
to be detected by recipients in Australia.
Option 2 - Retain WGS84 as
a normative reference in the CAP-AU Profile, and add the GDA94 as an additional
Normative Reference plus add a note into the AREA element to highlight there is
a difference in Australia and explain what is actually used in
Australia.
Can you please forward any
comments or observations for consideration to the EM-TC and Profiles SC email
lists.
Greg
Trott
CAP-AU
Project Manager
Australian
Government Attorney-General's Department
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