[GRASS5] projection-transformation

Markus Neteler neteler at geog.uni-hannover.de
Tue Jan 16 17:03:38 EST 2001

Hi all,

a short update on our map shift problems here...

On Thu, Jan 11, 2001 at 06:20:34PM +0000, Michel Wurtz wrote:
> Markus Neteler wrote:
> > 
> > While thinking about the problem I should describe the input dataset better:
> > The students have digitized the country's border from a 1:500000 paper map
> > of Lower saxony. Problem with this map is:
> >  - the map is projected in Lambert Conical projection (10:30E meridian)
> >  - the border's coordinates are given in degree, nothing else
> >  - ellipsoid is "international"
> Well... Lambert conical... that probably means that the meridian
> lines converge. A test : look if distance between the 6°E and the
> 11°E meridian are the same at the top and the bottom of the map.

Well, thi is not the case - fully non-linear shifts.
> I presume this is not the case.  Use the your Arc/info tic marks
> and convert the lat/long values to meters in the lambert conical
> projection used by the map (conformal or equal area projection ?
> what are the projection's central parallel or the two parallels
> used ?).
> Then you can simply use an afine transformation to transform lat/long
> to X,Y ; import the data in Grass in Lambert and reproject them (using
> the same lambert projection parameters) in lat/long.
> But I'm afraid that the non-linearity of the parallels/meridians
> introduced some irreversible glitch during the numerisation process
> (I guess also that the EMQ obtained when the students have referenced
> and the map at the beginning of the numerisation process was a little
> big when converted to km !)

I have extracted the tics out of the E00 file. They are exactly
where they should be. Then I scanned part of the map, rectified
affine with i.rectify - the drawn grids are matching well.

Solution: The students *must* have had a very bad day. I cannot 
reproduce that anything is wrong with m.in.e00. :-)

But we should put an eye on that. Helena, did you find anything
new here? Something which could be reproduced?

THank you all for your time and commments on this,


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