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Subject: Question regarding use of WGS84 reference system in CAP standard [SEC=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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