Plate Carre with latitude shift

[Ed McNierney]

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

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On Behalf Of Espen Isaksen
Sent: Friday, January 18, 2008 11:17 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Plate Carre with latitude shift

Hi!

Sorry for not answering this for a day or so. First of all thanks to
Ed and Frank for helping me out on this one!

What you explain is what I figured happened. But I still do not quite
understand why proj cannot transform my coordinates to the new system
with the latitude shifted. I have read one similar e-mail on this list
from 2005(although I cannot find it now) which asked the same
question.

Espen

On 17/01/2008, Ed McNierney <ed at topozone.com> wrote:
> Espen -
>
> Yes, that should be what you expect.  If you change the latitude of true
> scale, then you're changing the relationship between the size of a
> degree of longitude at a given latitude.  Normally the line of 60
> degrees North latitude is "stretched" horizontally to be larger than it
> should be.  It is displayed so that it appears that one degree of
> longitude (at that latitude) is the same size as one degree of longitude
> at the Equator (instead of being only half as long).  That makes it
> appear to be the same size as a degree of latitude and therefore you can
> easily interpolate coordinates because they are the same size.
>
> When you change the latitude of true scale to 60 degrees North, you are
> making every degree of longitude equal 0.5 degrees of latitude, and you
> can no longer treat them as simple X/Y coordinates to read latitude and
> longitude.  You need to keep in mind that you are really using a
> different projection, so your coordinate system is going to be
> different.
>
>         - Ed
>
> -----Original Message-----
> From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
> Behalf Of Espen Isaksen
> Sent: Wednesday, January 16, 2008 5:41 PM
> To: MAPSERVER-USERS at LISTS.UMN.EDU
> Subject: Re: [UMN_MAPSERVER-USERS] Plate Carre with latitude shift
>
> To further discuss this topic. I created a map of Europe in a regular
> EPSG:32662(proj=eqc) and one map of Europe with a latitude shift to 60
> degrees north:
>
> Map of Europe in 32662:
> http://bildr.no/view/143324
>
> Map of Europe with latitude shift:
> http://bildr.no/view/143325
>
> The latter is close to what I want. However, if I move my zoom in to
> some part of the nordic countries, the decimal degree coordinates will
> no longer correspond with my coordinates in my 32662 projection with
> the latitude shift. Why is this?
>
> Espen
>
>
> On 16/01/2008, Espen Isaksen <espen.isaksen at gmail.com> wrote:
> >  > EPSG:4326 is a geographic coordinate system, and the coordinates
> are in
> > > decimal degrees.  +proj=eqc is a projected coordinate system and the
> > > coordinates are meters.  The location (-180,90) would be roughly
> > > (-20000000,100000).
> > >
> > > Essentially eqc (ie. equidistant cylindrical or Plate Carree) is
> just
> > > a rescaling of EPSG:4326 from degrees to meters.
> > >
> >
> > Ok I understand this.
> >
> > > I'm afraid I just don't get what you hope to accomplish with
> +lat_ts=60.
> > > Are you hoping to get one projected meter being one meter on the
> ground
> > > at 60N as opposed to it being one meter at the equator as is the
> default?
> > > I'm not aware of proj=eqc supporting any such option.
> >
> > Yes I am trying to change the projection such that it gets less
> > distorted in areas near the north pole. I tried to set the mapfile
> > like this:
> >
> > EXTENT 593674.73 6669772.29 594581.49 6670417.57
> > PROJECTION
> >      "+proj=eqc +lat_ts=60 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84
> > +datum=WGS84 +no_defs +units=m"
> > END
> >
> > which seems to create an quite ok map. Much better than setting
> > +lat_ts=0. Although I do not understand why Mapserver creates a map
> > over a different area than what my original decimal degree areas
> > indicates(I used proj to convert them)?
> >
> > By the way my original decimal degree coordinates are:
> > 10.6661 59.9155 10.6824 59.9213
> >
> > Perhaps what I am doing is just nonsense...
> >
> > Espen
> >
>