[Proj] old german topo

John Smart john.smart at caris.com
Sun Jul 25 17:51:29 EDT 2004


Please see below...

> Thanks! I tried it and appears allright. I registered the Mestichblatter
> 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.

Having done this registration, how good is it? You need to ask at this
point, because if you are getting significant differences at this stage,
then clearly you won't get any better after you reproject and do the datum

Suggestions: point (in Grass or whatever) to a grid intersection that's
shown on the map; now what is the typical size of difference that you get
between the image of the intersection, and the nominal position? Do the
errors tend to be in any particular direction (i.e. systematic error), or
are the errors random? Also do the same test on some of your map's edge
ticks which show the nominal geographic positions. (If you are getting
different results between the grid intersections and the geographic values
then that suggests the projection is not what we are assuming).

For the registration quality, my guess is that you would be doing well to
have most errors within 0.2mm at map scale, i.e. within 5m on the ground for
your map. With the map-drawing process, printing and material aging, results
can't be expected to be too much better than that. The scan resolution may
also plays a part: I take it your scanned map has finer resolution than the
.png you sent out the other day?


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