Plate Carre with latitude shift

[Ed McNierney]

Frank -

My comment to Espen, "The lat_ts parameter is the latitude of true scale, and causes that latitude to be drawn at true scale, reducing distortion at that latitude", seems in conflict with your comment "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."

It sounds like I am misunderstanding what lat_ts does.  If it doesn't do what I thought (cause 60N to be the latitude of true scale, where one meter in map units equals one meter on the ground), what does it do?

     - 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 Frank Warmerdam
Sent: Wednesday, January 16, 2008 11:07 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Plate Carre with latitude shift

Espen Isaksen wrote:
> Ok, I set my mapfile like this:
> 
> EXTENT 10.6661 59.9155 10.6824 59.9214
> PROJECTION
>     "+proj=eqc +lat_ts=60 +lon_0=0 +x_0=0 +y_0=0 +datum=WGS84"
> END
> 
> and this is what I have had before:
> 
> EXTENT 10.6661 59.9155 10.6824 59.9214
> PROJECTION
>     "init=EPSG:4326"
> END
> 
>>From my understanding the former mapfile should give the same map as
> the latter, but the east-west coordinates should be pushed together as
> this formula would indicate: x=lon*cos(lat) where lat is 60 and the
> formula then gives x=lon*0.5?
> 
> All I get is a blank map, so I suppose there is something I have misunderstood?

Espen,

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.

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.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org