# [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,

Markus

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