[Proj] epsg parameters

Wed Apr 2 09:29:47 PDT 2008

Folks,

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?

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.

If this were simpler, what would consulting Cartographers and Geodesists do for a living?

Cliff Mugnier
LOUISIANA STATE UNIVERSITY

________________________________

From: proj-bounces at lists.maptools.org on behalf of Paul Kelly
Sent: Wed 02-Apr-08 11:24
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] epsg parameters

On Wed, 2 Apr 2008, Frank Warmerdam wrote:

> The problem is that if EPSG offers more than one transformation to
> WGS84 for a datum, the automated translation gives up and offers none
> of them preferring for the user to make the decision themselves.
>
> I suspect, given sufficient investigation, you would find that EPSG
> offers more than one transformation for the problem datums.
>
> I'm not saying this is a good decision in the automated translation.

I tend to think it's the best you can do given the situation. We have had
a similar/related issue in GRASS, and the current kludgy solution is to
store our own database of datum transformation parameters along with extra
information on where geographically the transformation is valid and the
accuracy of it. When importing a co-ordinate system with ambiguous datum
information the user is then presented with this choice and forced to make
it.

I think it's in general a bad idea to include datum transformation
information without any user interaction, because many users have the
misconception that there is somehow a "correct" set of parameters which
will always give them accurate results. When of course, as we've seen
discussed on this list many times, accurate datum transformation is very
much a localised thing: a transformation that gives accurate results in
one region of a country may give very inaccurate results in another
region, and a transformation that covers the whole country is in general
not very accurate anywhere.

One minor complication that I don't think I have ever seen given much
discussion (and the main reason I'm sending this mail) is that some
classical datums have recently been redefined by the relevant national
mapping agencies in terms of their relationship to WGS84 (see Note 1) In
this situation then (in my understanding) there is one and only one
"correct" set of datum transformation parameters, and it *would* be valid
to include this without any user interaction (I'm not necessarily talking
about generation of the PROJ epsg file, just in general).
I was wondering though did anybody know if there was any standard way of
specifying (for example in the WKT format, or in the EPSG database) that a
set of datum transformation parameters is "exact" in this way?

Paul

Note 1: E.g. OSGB36 in the UK
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide5.html
Although this uses a grid-based datum shift rather than a set of
parameters, the prinicple of the datum having an exact relationship with
WGS84 is still the same.
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