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Subject: Re: [emergency] RE: [emergency-gis] Re: [emergency] Groups - CAP 1.1 Subcommittee Draft (CAP_1.1-draft_1-4-05) uploaded
Art -
Thanks for the response. Yes, the emails have been about how to express a
typographic convention. I was trying to point out that it would be better
to follow international convention. Sorry you could not find the
documents - they can be a bit obscure.
Anyway, as you know and is referenced in CAP, the normative reference for
CRS (Coordinate Reference System) information is the EPSG (www.epsg.org).
The EPSG provides a geodetic parameter database of CRS representations. As
that organiztion states: The EPSG geodetic parameter dataset is a
repository of parameters required to:
1. identify coordinates such that those coordinates describe position
unambiguously. This is through a coordinate reference system (CRS)
definition.2. define transformations and conversions that allow coordinates to be
changed from one CRS to another CRS. Transformations and conversions are
collectively called coordinate operations.
Within the EPSG database is CRS definition 4236 (WGS 84 geographic 2d CRS.
I believe that this is the one referenced in CAP 1.0. I do not have the
document in front of me, so I apologize if I am off-base.
Coordinates are unambiguous only when the unit and unit representation
used are identified. For geographic CRSs with units of degrees, the degree
representation information is no longer explicitly provided as part of the
EPSG CRS description. EPSG geographic CRSs such as codes 4326 and 4267
allow (and require) the degree representation to be defined for the user
by the coordinate data supplier.
Further, the values in a WGS 84 geographical 2D CRS are given in decimal
degrees. For areas straddling the 180 meridian, the "west" longitude will
have a higher value than the "east" longitude.
Therefore, best practices would dictate that the coordinate data supplier
provide the information as to how the decimal degrees are expressed rather
than the spec limit the expression of those values. For example, decimal
degrees of longitude could be expressed in the range of 0 to 360 or -180
to +180.
Within this context, "signing" a value is restricting the coordinate data
supplier. Do we wish to do this?
Cheers
Carl
> At 11:54 AM -0500 1/18/05, David Danko wrote:
>> I thought CAP used WGS coordinates (unprojected) so we wouldn't have
>> the
>>problem with differnt projections.
>
> As I understand it, the question here is about how best to describe a
> particular typographic convention, not about reopening the whole
> projections question.
>
> Carl, I looked at the OGC documents and couldn't find a direct
> reference that seemed to go to this particular point... and adopting
> the whole document looked like a much broader proposition... although
> I'm clearly not the most qualified person to go looking through GIS
> specs.
>
> If you're seriously suggesting the TC consider adopting these
> documents entire, seems like that could be discussed, although I
> wonder if it might be more appropriate in the CAP 2.0/EDXL timeframe.
> Otherwise, would you like to propose a simple citation that would
> address this particular point?
>
> - Art
>
>>
>>DAve
>>
>>