[GRASS5] projection-transformation

William L. Baker BakerWL at uwyo.edu
Thu Jan 11 12:44:11 EST 2001

Hi Markus,

I guess I wonder if you want the map in GRASS to be in
Lambert conical.  That's not necessary of course. 

It seems right to use PROJECTDEFINE to make sure
the Lambert stuff is all specified correctly after digitizing.
Check it using DESCRIBE.

Then, you could use PROJECT to turn it into a UTM
map before bringing it into GRASS, since GRASS handles
UTM well.  That's what I do, but I typically only work with
GRASS maps in UTM.  Here's what I do with PROJECT:

	ZONE xx

Substitute for "in" "out" and "xx" 

But, you have probably done this!

Bill B.

-----Original Message-----
From: Markus Neteler [mailto:neteler at geog.uni-hannover.de]
Sent: Thursday, January 11, 2001 10:24 AM
To: grass5 at geog.uni-hannover.de
Subject: Re: [GRASS5] projection-transformation

Hi Michel,

On Thu, Jan 11, 2001 at 03:49:29PM +0000, Michel Wurtz wrote:
> Markus Neteler wrote:
> > 
> > Hi Michel,
> > I have received another SHAPEfile with borders which matches quite good
> > to my other data. So three options are there:
> > 
> >  - mistake done by the students
> >  - e00 needs Xshift and Yshift set
> Well, this is probably easy to handle, but I need some example for
> this (the questions is : does we apply shift before or after scaling ?)
> >  - general ArcInfo problem (btw: it was PC3.5, not 8)
> For the two other options, I can't do anything ;-)

I have investigated some more hours...

Here the results:
 - we have used "projectdefine" to add a projection in ARCINFO8
   (using the PC-ARCINFO3.5 e00 file)

-> import into GRASS is identical. The misplacement is *non-linear*.

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"

Now the students have referenced the map to lat/long and digitized.
The misplacements are like that (compared with an other digital governmental
dataset and the GSHHS shorelines):

 6°         7°         8°         9°        10°        11°
 +          +          +          +          +          +  53:30°
  north sea    ***(islands)  ***                  -----         
         ***   ----------------   /--------------/       \
  *** --------/               |  /                        \
 /--/                        \---                          |
 |   shift: 2.5km to E                                     |  + 53 E
 |          500m  to S                             
 |                                                 shift: +/- 0
/                   LOWER SAXONY (northern Germany)
|                                                              + 52 E
|                                                            |
\    shift: 1km to W (!)                                     |
 \          0 to S                                           /
  \                                                          /
   ------------\             /-----\                      /
                \           /       \                     /
                 \         /         \                     /
                  ---------|          \                   /
                                       \                 /
                                         shift: 1.5km to E      + 51 E
                                                2km   to S

Let me say: I am slightly confused.
Perhaps the problem is that the map's projection is Lambert, but GRASS
et al. treat is as Geographic (lat/long)? But, then, how to get in
a map projection in Lambert if only lat/long coordinates are printed on
that map?

I am quite in the dark, 
help is appreciated (especially to save the digitizing work)

 Thanks for listening!


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