Darrell -
Nicely stated. Foundation for an ongoing excellent, consensus building
discussion.
Carl
From: Darrell O'Donnell
Sent: Monday, November 21, 2011 12:14 PM
To: Gary Ham ; Carl
Reed
Cc: Pete O'Dell ; Leinenweber, Lewis ; Trott,
Gregory ; Elysa Jones ;
;
Subject: Re: [emergency] Question regarding use of WGS84 reference
system in CAP standard [SEC=UNCLASSIFIED]
Gary – noted on the signed message issue – it would have to happen at the
originator level. I am betting Carl was considering the issuer and server to be
one and the same, which I would agree with for the purposes of this discussion.
Certainly the issuer and server don't need to be the same, but Carl's point is
that it is quite easy to share out both WGS84 and coordinates in another
CRS.
Your email raises a very good discussion point though – the burden is on
the issuer.
Carl's point applies to the system that is issuing the alert, especially
where digital signing is involved. To me it makes sense since most systems that
use non-WGS84 coordinate systems nowadays can handle WGS84 as it has become a
leveller. So, adding "hey this is my local CRS" parameter value still makes
sense here.
It's a good approach – I am betting if a system can handle an arbitrary
CRS, it can handle WGS84 as well. The converse is not true from what I have seen
in the wild.
At the CAP v1.2 level I can see adding a GML-compliant valueName/value that
allows extension to the play.
Here's a question though - on the future side (CAP v2+) when we allow for
CRS data to be shared (that's good) will we require everyone to also share out a
WGS84 common coordinate as well? To me, any part of CAP that isn't common
becomes very dangerous. I can picture an alert that uses a new local state-plane
coordinate system being totally useless to a receiving system that only
understands WGS84.
Cheers,
Darrell
From: Gary Ham <>
Date: Mon, 21 Nov 2011 14:02:43 -0500
To: Carl Reed <>
Cc: Pete O'Dell <>,
"Leinenweber, Lewis" <>,
Darrell O'Donnell <>,
"Trott, Gregory" <>, Elysa
Jones <>,
<>,
<>
Subject: Re: [emergency] Question regarding use
of WGS84 reference system in CAP standard [SEC=UNCLASSIFIED]
Not
if the messages are signed. The data could then not be changes without
invalidating the signature. Changed messages are new messages. The basic
message cannot be changed, although what the user sees can be changed as needed.
Gary Ham
http://grandpaham.com
703-899-6241
On Nov 21, 2011, at 1:45 PM, Carl Reed wrote:
Providing the ability to specify additional coordinate reference systems
would not change any of the existing implementations. Any code necessary to
check for additional CRSs would be trivial. And any actual coordinate
conversions should happen on the server side where the alerts are generated
and not on the client side (although many clients now also support easy access
to coordinate transformations.)
Regards
Carl
From: Pete
O'Dell
Sent: Monday,
November 21, 2011 11:19 AM
To: Leinenweber, Lewis ; Carl Reed ; Darrell O'Donnell ; Trott, Gregory
Cc: Elysa Jones ; ;
Subject: RE:
[emergency] Question regarding use of WGS84 reference system in CAP standard
[SEC=UNCLASSIFIED]
I’m not sure
I’m following and understanding all the different technical threads.
From a
usability standpoint, the planet is shrinking (digitally) and many
people/organizations in many countries want to be able to consume alerts that
are relevant to their global organizations outside of physical country
boundaries.
Any resolution
to this issue that is transparent to the alert recipient without an
incredible amount of additional code and business rule processing to resolve
differences between the various coordinate systems being used would be
appreciated and help this objective long term. CAP has been an
incredible purveyor of information.
From: [mailto:] On Behalf Of Leinenweber,
Lewis
Sent: Monday,
November 21, 2011 12:50 PM
To: Carl Reed; Darrell O'Donnell; Trott,
Gregory
Cc: Elysa
Jones; ;
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: [mailto:] On Behalf Of Carl Reed
Sent: Mon, 21 Nov 2011 11:38
AM
To: Darrell
O'Donnell; Trott, Gregory
Cc: Elysa Jones; ;
Subject: Re: [emergency] Question regarding
use of WGS84 reference system in CAP standard
[SEC=UNCLASSIFIED]
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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