Plate Carre with latitude shift

[Espen Isaksen]

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
> >
>