<HTML dir=ltr><HEAD><TITLE>Re: [Proj] epsg parameters</TITLE>
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<DIV dir=ltr><FONT face="Times New Roman" color=#000000 size=2>Folks,</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Things change. That includes Datums. Even National publications of "official" datum transformation parameters change from time-to-time. WGS84, ITRFxx, etc., are various realizations of the shifting continents. We often find 7-parameter transformations being published without any indication of which "sense" of rotations is supported. The only way to have a good guess is when a test point is provided by a National government. Nevertheless, as datums evolve, so do the transformations. How do you keep up?</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Check, and re-check every time you need such a transformation. Usually, but not always, a national govt. will publish a notice that things have changed.</FONT></DIV>
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<DIV dir=ltr><FONT size=2>If this were simpler, what would consulting Cartographers and Geodesists do for a living?</FONT></DIV>
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<DIV dir=ltr><FONT size=2>Cliff Mugnier</FONT></DIV>
<DIV dir=ltr><FONT size=2>LOUISIANA STATE UNIVERSITY</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> proj-bounces@lists.maptools.org on behalf of Paul Kelly<BR><B>Sent:</B> Wed 02-Apr-08 11:24<BR><B>To:</B> PROJ.4 and general Projections Discussions<BR><B>Subject:</B> Re: [Proj] epsg parameters<BR></FONT><BR></DIV>
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<P><FONT size=2>On Wed, 2 Apr 2008, Frank Warmerdam wrote:<BR><BR>> The problem is that if EPSG offers more than one transformation to<BR>> WGS84 for a datum, the automated translation gives up and offers none<BR>> of them preferring for the user to make the decision themselves.<BR>><BR>> I suspect, given sufficient investigation, you would find that EPSG<BR>> offers more than one transformation for the problem datums.<BR>><BR>> I'm not saying this is a good decision in the automated translation.<BR><BR>I tend to think it's the best you can do given the situation. We have had<BR>a similar/related issue in GRASS, and the current kludgy solution is to<BR>store our own database of datum transformation parameters along with extra<BR>information on where geographically the transformation is valid and the<BR>accuracy of it. When importing a co-ordinate system with ambiguous datum<BR>information the user is then presented with this choice and forced to make<BR>it.<BR><BR>I think it's in general a bad idea to include datum transformation<BR>information without any user interaction, because many users have the<BR>misconception that there is somehow a "correct" set of parameters which<BR>will always give them accurate results. When of course, as we've seen<BR>discussed on this list many times, accurate datum transformation is very<BR>much a localised thing: a transformation that gives accurate results in<BR>one region of a country may give very inaccurate results in another<BR>region, and a transformation that covers the whole country is in general<BR>not very accurate anywhere.<BR><BR>One minor complication that I don't think I have ever seen given much<BR>discussion (and the main reason I'm sending this mail) is that some<BR>classical datums have recently been redefined by the relevant national<BR>mapping agencies in terms of their relationship to WGS84 (see Note 1) In<BR>this situation then (in my understanding) there is one and only one<BR>"correct" set of datum transformation parameters, and it *would* be valid<BR>to include this without any user interaction (I'm not necessarily talking<BR>about generation of the PROJ epsg file, just in general).<BR>I was wondering though did anybody know if there was any standard way of<BR>specifying (for example in the WKT format, or in the EPSG database) that a<BR>set of datum transformation parameters is "exact" in this way?<BR><BR>Paul<BR><BR>Note 1: E.g. OSGB36 in the UK<BR><A href="http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide5.html">http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide5.html</A><BR>Although this uses a grid-based datum shift rather than a set of<BR>parameters, the prinicple of the datum having an exact relationship with<BR>WGS84 is still the same.<BR>_______________________________________________<BR>Proj mailing list<BR>Proj@lists.maptools.org<BR><A href="http://lists.maptools.org/mailman/listinfo/proj">http://lists.maptools.org/mailman/listinfo/proj</A><BR></FONT></P></DIV></BODY></HTML>