[Proj] Re: PROJ 4.6.1RC2 Available

[Mikael Rittri]

Hello,
In the ticket, Frank wrote: 

> I'd like some supporting documentation indicating 
> that ETRS89 is essentially equivalent to WGS84.

May I quote myself:

> However, for most practical purposes, ETRS89 can be regarded as 
> identical to WGS84 ... The difference is mainly caused by the 
> continental drift since 1989, which moves Europe (and ETRS89) 
> about 2 cm/year towards east-northeast. Differential GPS in Europe 
> will (primarily) give coordinates in ETRS89, but such coordinates 
> are often called "WGS84 coordinates", in a loose sense of the word. 
  (From the Carmenta Engine manual.)

My primary source is the text by the British Ordnance Survey,
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf
or 
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/
which, by the way, I find wonderfully clear and easy to read. 
ETRS89 is described in section 4.2.4.  

I admit that the guide does give some (time-dependent) 7-parameter 
transformations between WGS84 and ETRS89, in section 6.5.  But as I 
understand it, these would be needed only for special scientific 
applications (perhaps interferometry with very long baselines, where 
observatories on different continents have to cooperate.)  For everyday 
mapping, ETRS89 is used.  For positioning, if you use GPS with differential 
correction, you will also get ETRS89.  And if you use GPS without differential 
corrections, then you will indeed get WGS84 instead of ETRS89, but the accuracy 
will be worse than the (currently) half-metre difference between the datums, 
so it will not matter anyway. 

Best regards,
--
Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 Göteborg
Visitors: Sankt Eriksgatan 5
SWEDEN
Tel: +46-31-775 57 37
Mob: +46-703-60 34 07 
mikael.rittri at carmenta.com
www.carmenta.com
-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Frank Warmerdam
Sent: den 22 augusti 2008 17:04
To: Maciej Sieczka
Cc: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Re: PROJ 4.6.1RC2 Available

Maciej Sieczka wrote:
> Hi,
> 
> Is a fix for "ETRS89-based CRSs missing datum definition" included?
> 
> [1]http://trac.osgeo.org/proj/ticket/11

Maciek,

No, that issue is not resolved yet (as is implied in the ticket).

Best regards,
-- 
---------------------------------------+--------------------------------
---------------------------------------+------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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