>I can check.
>
>Carl
Both Galileo and GLONASS use datums which are distinct from
WGS84. PZ-90, which is the datum used by GLONASS, does differ
from WGS84, though the difference is minor (Wikipedia puts
it as "less than 40cm in any given direction").
In some applications, 40cm is significant, and if you can eliminate
an error by noting that the reference datum was PZ-90 rather
than WGS84, it seems useful to me personally to do so.
To back up a step, I have to ask what we think we're getting
by using a URI here. This is clearly not a URI intended to be
used to trigger protocol processing; it is instead a way of
capturing the context of the data. For those of us who use
GPS and systems designed to work with GPS, lat, long, altitude
and WGS84 are the syntax and context for location. This
group could agree to mint a URI scheme that works only
in that context and it would have a very simple syntax.
It could instead have a slightly more general syntax and
mechanism for indicating context (the parameter indicating
datum). I think the latter is more in keeping with the purpose
of URIs. Shorn of even the possibility of using new or different
datums, I think we will either see other URI schemes,
the same URI schemes used with different datums (thus introducing
invisible differences in reference coordinates). Though I
usually fall into the "strings are cheap" view of URI schemes,
having a URI proliferation like geo:, geoGLONASS:, geoWGS84-2009:
etc strikes me as unlikely to be a long term win. And having
folks use the same URI scheme with different but unmarked
datums makes me very concerned indeed.
The parameter method suggested before is easy to specify,
relatively cheap to parse, and it meets my personal test
for simplicity. I understand it doesn't meet others, but
speaking personally, I think the resistance to it doesn't
match the real likelihood of problems. Unless someone
builds a parser without reading the spec or referring to
a standard URI library, there's a fair chance they'll get it
right.
My two cents,
Ted
>
>----- Original Message -----
>From: "Alexander Mayrhofer" <alexander.mayrhofer@nic.at>
>To: "GEOPRIV" <geopriv@ietf.org>; <vcarddav@ietf.org>
>Sent: Wednesday, June 03, 2009 7:18 AM
>Subject: Re: [Geopriv] [VCARDDAV] The "geo" URI draft
>
>
>>
>>
>>> Also, the forthcoming Galileo system:
>>>
>>> <http://en.wikipedia.org/wiki/Galileo_positioning_system>
>>>
>>> will not, I understand, use WGS84.
>>
>> I don't claim to be an expert in reference systems / geopotential models
>> etc. so please take the following information with caution (Carl? ;)
>>
>> I couldn't find any information about the reference system that Galileo
>> uses on the wikipedia page. However, a bit of research shows that
>> Galileo seems to use ITRF ("International Terrestrial Reference Frame"),
>> which, however, seems to be practically identical to WGS84, according to
>> http://www.dqts.net/wgs84.htm (web site by EUROCONTROL). Also, this
>> http://www.cambridgeconference.com/2007_conference_information/Conferenc
>> e%20proceedings/w2_5_cross.pdf presentation notes on slide 15 that
>> "WGS84 and IRTF are aligned".
>>
>> Which means that Galileo will use a reference system that is practically
>> identical to WGS84.
>>
>> Again, Disclaimer: i'm not an expert in this field.
>>
>> Alex
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