[Proj] What about datum shift via direct projection?

Jose Gonçalves jagoncal at gmail.com
Fri Dec 12 08:56:45 EST 2008

In Portugal we had a situation of that kind. There was an old datum (Datum
Lisboa) for which we had a local Transverse Mercator projection. A
reobservation of the triangulation resulted in a new datum (Datum 73) that
intended to replace the old one. It was decided to do a new projection,
keeping the same central point, but applying a small false easting and
northing (aprox. 180.6 m, -87.0 m) in order to create similar coordinates.
The average of absolute differences between old and new coordinates are
smaller than 2 meters and maximum difference is  5 meters.

Although this might have some advantages,  it resulted in many situations of
confusion. Around 1990 many people started GIS projects, but were not much
aware of map projections, and even less of geodetic datum concepts. Some
institutions adopted the new system but some kept the old one. Many times
old and new coordinates were simply mixed, losing information about their
origin. This results in serious problems in large scale mapping.
Now, in the transition to ETRS89 datum, the Portuguese Geographic Institute
decided to keep the TM projection and adjusted the central point (instead of
applying the small false easting/northing) in order to create a very similar
grid. The transition to a new similar system must be made carefully, always
keeping metadata about reference systems and defining rigorous
transformation methods.

Jose' A. Gonçalves
University of Porto - Science Faculty

2008/12/11 Mikael Rittri <Mikael.Rittri at carmenta.com>

> 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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