Plate Carre with latitude shift

[Ed McNierney]

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
>