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Subject: RE: [emergency] Question regarding use of WGS84 reference system in CAP standard [SEC=UNCLASSIFIED]
Darrell, Carl et.al. – I would like to add some additional thoughts to the discussion
regarding the ability to support coordinate reference systems in EDXL messages. A new, soon to be released, OASIS Committee Specification Draft
(CSD) called EDXL GML Simple Features profile (EDXL-GSF), based on OGC and ISO
international standard location-based GML schema, allows one to specify and use
any known coordinate reference system (CRS) for a given location. Thus, while
the default CRS may be WGS84, a CAP message, for example, using EDXL-GSF
profile for location geometry (point, polygon, envelope, etc), could contain
locations using GDA94 with clarity and without ambiguity. For additional
details and description see the documentation excerpts from the EDXL-GSF
profile schema (and by reference, also GML 3.2.1 standard schema) provide the
description of its use in practice. The purpose of the EDXL-GSF profile is to provide a
standards-based way to provide location information in a message. To
incorporate use of EDXL-GSF profile would, of course, require each EDXL message
standard Subcommittee to review and consider the path to achieve integration of
this supporting standard (when it is eventually published). edxl-gsf-base.xsd (ref GML schema: geometryBasic0d1d.xsd): ---------------------------------- pos --> DirectPositionType: -------------- “Direct position instances hold the coordinates for a position
within some coordinate reference system (CRS). Since direct positions, as data
types, will often be included in larger objects (such as geometry elements)
that have references to CRS, the srsName attribute will in general be missing,
if this particular direct position is included in a larger element with such a
reference to a CRS. In this case, the CRS is implicitly assumed to take on the
value of the containing object's CRS. If no srsName attribute is given, the CRS
shall be specified as part of the larger context this geometry element is part
of, typically a geometric object like a point, curve, etc.” posList --> DirectPositionListType: -------------- “posList instances (and other instances with the content model
specified by DirectPositionListType) hold the coordinates for a sequence of
direct positions within the same coordinate reference system (CRS). If no
srsName attribute is given, the CRS shall be specified as part of the larger
context this geometry element is part of, typically a geometric object like a
point, curve, etc. The optional attribute count specifies the number of direct
positions in the list. If the attribute count is present then the attribute
srsDimension shall be present, too. The number of entries in the list is equal
to the product of the dimensionality of the coordinate reference system (i.e.
it is a derived value of the coordinate reference system definition) and the
number of direct positions.” ---------------------------------- Hope this helps and doesn’t muddy the waters of the discussion. Lew Leinenweber SE Solutions, Inc. 301.351.4485 From:
emergency@lists.oasis-open.org [mailto:emergency@lists.oasis-open.org] On
Behalf Of Carl Reed 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 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" <Gregory.Trott@ag.gov.au> 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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