[GRASS5] projection-transformation

Michel Wurtz mw at teledetection.fr
Thu Jan 11 13:20:34 EST 2001

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.

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 !)

Michel WURTZ - DIG - Maison de la télédétection
               500, rue J.F. Breton
               34093 MONTPELLIER Cedex 5

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