Plate Carre with latitude shift

[Espen Isaksen]

Hi again!

This is what I try to do. I understand that I am now using a projected
coordinate system with 60N as my latitude of true scale.

First I create a mapfile using EPSG:4326 and output a map of a smaller
area from this mapfile. Then I take the same extent as as I use in my
geographic mapfile and put those coordinates in a textfile. Then I use
the following command to transform these coordinates to my new
projected coordinate system:

proj +proj=eqc +lat_ts=60 +lon_0=0 +x_0=0 +y_0=0 + ellips=WGS84
+datum=WGS84 +no_defs +units=m < test.txt

I get my new coordinates as my output. I then create a mapfile, now
using the projected coordinate system instead of EPSG:4326 and using
my new coordinates for my extent. Creating an output map for this
mapfile creates a map from a very different area than my former
mapfile using 4326.

Doing the same, just converting to UTM32 instead creates a map of the
same area as 4326 does. This leads me to believe that I am doing the
right thing with my mapfile.

Espen

On 18/01/2008, Ed McNierney <ed at topozone.com> wrote:
> Espen -
>
> I don't quite understand what you mean when you say "proj cannot transform my coordinates".  Could you explain exactly what it is you're trying to do, and what you think PROJ is or is not doing?
>
> Please keep in mind that you have moved from a geographic coordinate system to a projected coordinate system, and that that is a material and substantial change.  It's not simply a matter of "shifting the latitude" but of actually projecting your data to an equirectangular projection with 60N as the latitude of true scale.  PROJ can certainly also transform coordinates in that projection back to a geographic coordinate system, but that's a separate task.  You're asking MapServer to create a projected output map, and that output map's coordinate system will, of necessity, be the projected coordinate system.
>
> If you also need to display coordinates in a different coordinate system (such as latitude/longitude) then that is an additional task that you will need to do separate from the projection of your map image.
>
>      - Ed
>
> Ed McNierney
> Chief Mapmaker
> Demand Media / TopoZone.com
> 73 Princeton Street, Suite 305
> North Chelmsford, MA 01863
> ed at topozone.com
> Phone: +1 (978) 251-4242
> Fax: +1 (978) 251-1396
>