merc projection changing lat_0 has no effect

Ed McNierney ed at TOPOZONE.COM
Mon Mar 19 21:13:43 EDT 2007


David -

The Mercator family of projections includes the conventional Mercator
(projection on to a cylinder wrapped around the Earth at the Equator),
the Transverse Mercator (cylinder is wrapped around the poles and a
specified meridian), the Universal Transverse Mercator (just a special
case of the former), and the Oblique Mercator, in which the cylinder is
wrapped around the Earth at another location.

What Ryan is trying to do is create an Oblique Mercator (assume that the
value he wants to use for lat_0 is not 90 degrees!).  That's fine, but
it's a complex projection, as it is really more or less the general case
for all Mercator-type projections (with limitations).

There are a few national grid systems based on Oblique Mercator
projections, and they are used in satellite imaging (Space Oblique
Mercator) applications.

     - Ed

P.S. Even if you're not an Oblique Mercator user, Larry's article is
excellent and well worth reading!

Ed McNierney
TopoZone.com

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of percy
Sent: Monday, March 19, 2007 8:43 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] merc projection changing lat_0 has no
effect

http://topomaps.usgs.gov/drg/mercproj/index.html

The way I read this is that you can ONLY change longitude, and that 
latitude is fixed at the equator. And the opposite holds true for 
Transverse Mercator...

Happy to be corrected if this is not the case :-)

I typically use Albers Equal Area and shift the lat/lon settings for 
maps of larger regions...


Ryan Ollerenshaw wrote:
> I am trying to reproject my map which is in the geographic projection
> 4326 to a Mercator projection, which works, however, when i change
> lat_0 to anything it has no effect.  I can change lon_0 and the images
> shifts appropriatly but for some reason lat_0 has no effect.
> 
> -Thank you.
> 

-- 
David Percy
Geospatial Data Manager
Geology Department
Portland State University
http://gisgeek.pdx.edu
503-725-3373



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