[Proj] What about datum shift via direct projection?
support.mn at elisanet.fi
support.mn at elisanet.fi
Tue Dec 16 00:46:04 EST 2008
Why not? There is no rule how to do datum shifts. If the
results are exactly the same, nobody will argue.
But I am assuming that there are some differences and
so the method should be mentioned with the datum
name, so that the results could be reproduced.
Mikael Rittri [Mikael.Rittri at carmenta.com] kirjoitti:
> Gerald wrote:
> > While there seems to be lull in the hot debate about separation of
> > church and state ... er ... datum and projection,
> > [...]
> > Thus, why is it so necessary to bind the two operations so tightly as done
> > in the proj.4 distribution? I cannot find a precedence for this concept
> This post is not specifically about the PROJ.4 design (so I changed
> the Subject line), but it is about how much datums and projections
> can and should be separated.
> There is method for datum shift that uses a direct projection.
> As an example, the old Swedish Grid is traditionally defined
> on the Swedish RT90 datum (ellipsoid: Bessel 1841) and using a
> Transverse Mercator projection with
> central meridian: 15° 48' 29.8" E
> scale factor: 1
> false easting: 1500000 m
> false northing: 0 m
> ( http://www.lantmateriet.se/templates/LMV_Page.aspx?id=4766&lang=EN )
> With this definition, one would need some datum shift method
> to transform between RT90 lon/lat and WGS84 lon/lat.
> However, a simpler method, now recommended by the Swedish Land Survey
> instead of a 7-parameter shift, is to start from the WGS84 datum, and than
> tweak the projection parameters a little: just use a Transverse Mercator
> central meridian: 15° 48' 22.624306" E
> scale factor: 1.00000561024
> false easting: 1500064.274 m
> false northing: -667.711 m
> ( http://www.lantmateriet.se/templates/LMV_Page.aspx?id=5197&lang=EN )
> A paper describing this technique is
> http://www.fig.net/pub/fig2006/papers/ps05_03/ps05_03_04_engberg_lilje_0670.pdf .
> So, I have some rather vague questions to the readers of this list:
> - What do you think of this technique?
> - Is anyone else using it?
> - Doesn't the technique imply that a projected coordinate system
> may have an ambiguous geographic coordinate system? For the Swedish Grid,
> I can think of the geographic coordinate system as RT90 lon/lat, if I use
> the traditional projection parameters. Or I can think of it as WGS84 lon/lat,
> if I use the direct projection instead.
> - If the correct answer to the previous question is "No, you fool", then what?
> If I wanted to express the Swedish Grid, datum-shifted by the direct projection,
> in Well-Know Text, then I would be forced to say that the geographic coordinate
> system is WGS84 lon/lat. But then the resulting CRS cannot be Swedish Grid,
> because Swedish Grid has traditionally RT90 lon/lat as its geographic coordinate
> I think direct projections for datum shifts are efficient and easy to
> use, and normally as accurate as a 7-parameter shift. But when I try
> to fit this method into the traditional framework that separates datum
> shifts and projections, and which insists that each projected CRS
> has a unique geographic coordinate system, I run into problems.
> Are these problems caused by inflexibility in the traditional framework?
> Or is the method of direct projection just weird?
> Or am I missing some good way to reconcile them?
> Best regards,
> Mikael Rittri
> Carmenta AB
> Box 11354
> SE-404 28 Göteborg
> Visitors: Sankt Eriksgatan 5
> mikael.rittri at carmenta.com
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