[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.

Regards: Janne.


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 
> with 
>    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 
>   system. 
> 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
> www.carmenta.com
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