[Proj] old german topo

[Hamish]

> > > I tried it and appears allright. I registered the Mestichblatter
> > > map using Grass 5.3 into the projection above with all the grid
> > > crossings(121 points), exported into tiff/tfw, reprojected into
> > > the Polish"System 92" (tmerc on GRS80) using gdalwarp and imported
> > > it into Grass mapset with a "System 92" 1:10000 topo.When the
> > > Mestichblatter is displayed over the "System 92" map it looks
> > > quite in sync but there are differences of about 3-15m in the
> > > location of even churches and few other buildings which I don't
> > > suppose to have moved. I can't see a clear tendency in these
> > > differences, they are in different directions. Do you think the
> > > projection used may not be the proper one? Or maybe I expect too
> > > much? I mean the map was prepared in '30s and it's only 1:25000 so
> > > it can't be any more accurate propably (?).
> 
> > I think you have to check the results step by step. The projection I
> > suggested may or may not be the right one!
> 
> > First, I am curious as to how you did the registration. 121 points
> > you say, but how was the image transformed? Although I'm not
> > familiar with Grass, one typical approach would be to have a
> > best-fit to those control points, in which case you don't expect the
> > image of any grid intersection to match exactly to its nominal
> > position (since error is distributed). A second approach would be to
> > use a rubber-sheeting transformation, where the images of your grid
> > intersections will be mapped exactly to the nominal positions, the
> > areas between grid intersections being stretched or shrunk. Usually,
> > I prefer the first approach, with an affine transformation,
> > especially if I think I know the projection.
> 
> I think I used the best method available in Grass - i.rectify with "3d
> order polynomial transformation matrix". With those 121 points the RMS
> was 6.24. Is there any better way? Or maybe some other software? There
> is no such thing like rubber-sheeting in Grass AFAIK.

fyi,

from the GRASS i.points help page:
http://grass.ibiblio.org/gdp/html_grass5/html/i.points.html

"A good rule of thumb is to mark at least 12 to 15 points which are
evenly distributed over the entire imagery group file in order to obtain
an accurate transformation equation for the rectification process. The
RMS error may increase with more points added, but the transformation
equation will be more accurate.

An RMS error of less than or equal to approximately one resolution unit
(pixel or cell) for the image being rectified is generally considered
acceptable."



from the GRASS i.rectify help page:
http://grass.ibiblio.org/gdp/html_grass5/html/i.rectify.html

"The polynomial equations are performed using a modified Gaussian
elimination method."



also note the i.points3 module:
"i.points3 allows the user to select either a polynomial, orthographic,
or LANDSAT-Thematic Mapper transformation."




Hamish