[Proj] What about datum shift via direct projection?

[Clifford J Mugnier]

In regard to "direct projection,"  it happens to be a very old method of changing datums.  With some variations, it is essentially how the European Datum of 1950 was developed by the U.S. Army Map Service (AMS) after WWII.  They used the UTM Grid to accomplish the result on the complex plane.  Designed by Geodesists at AMS, the paper and pencil work was done mostly by Europeans after the war.  ED50 covered (and covers) all of Western Europe and the entire Mediterranean Region.
 
It's not that elegant, but it DOES work, especially if you are not too concerned about the geoid.  Cartometrically speaking, it's "bang on."
 
Cliff Mugnier
LOUISIANA STATE UNIVERSITY

________________________________

From: proj-bounces at lists.maptools.org on behalf of Mikael Rittri
Sent: Thu 11-Dec-08 09:43
To: PROJ.4 and general Projections Discussions
Subject: [Proj] What about datum shift via direct projection?




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
SWEDEN
mikael.rittri at carmenta.com
www.carmenta.com
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