[Proj] What about datum shift via direct projection?
Mikael Rittri
Mikael.Rittri at carmenta.com
Thu Dec 11 10:43:08 EST 2008
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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