[Mapserver-users] Can't seem to get the projection correct

[Ed McNierney]

Daniel -

As I mentioned before, you are probably accustomed to seeing maps in the
Mercator projection.  Rather than fiddling with the image size, you
might want to look into having MapServer generate Mercator output
images.

The Mercator projection stretches the Y-dimension as compared to the
geographic projection.  The distance between parallels of latitude
increases as you move away from the equator.  At a Missouri-ish latitude
of 40 degrees North, one degree of latitude is 1.414 times taller in a
Mercator projection than in a geographic projection.  This would mean
that if your horizontal (longitude) scale were, say, 7 pixels per
degree, you would expect your vertical (latitude) scale to be 7 * 1.414
= 9.9 pixels per degree.

Since this happens to be exactly what we see in the MapInfo image you
sent (which is roughly centered on Missouri), I think it's pretty likely
that both your memory and your MapInfo maps are in the Mercator
projection.  If you set up MapServer to create Mercator output images I
suspect you'll match your MapInfo results without having to fudge the
image size (which isn't really correct anyway because the vertical scale
distortion is nonlinear and varies with the latitude).

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: (978) 251-4242  Fax: (978) 251-1396
ed at topozone.com


-----Original Message-----
From: Martin, Daniel [mailto:DMartin at erac.com] 
Sent: Tuesday, February 18, 2003 3:17 PM
To: mapserver-users at lists.gis.umn.edu
Subject: RE: [Mapserver-users] Can't seem to get the projection correct

I want to thank everyone for all the responses.

Daniel Morissette hit the nail on the head.  MapInfo and Mapquest are
indeed
doing some matter of x-scale adjustment to make maps "look better".  

I know this isn't scientific, but my maps in MapInfo look like I expect
them
too.  For instance, my home state of Missouri that I've seen on maps
thousands of times "looks" like I know it to look in MapInfo.  In
MapServer
it "looks squished".  I don't know how to express it any different than
that.  One way I've adjusted for it is to force a new aspect ratio in
the
<img > tag on the HTML template to "squish" the image back.  But I hate
that
solution.  It's not a good solution in my book.

I'm going to take a look into some things mentioned and see if I can get
anywhere.  I'll be sure to let you know how they turn out.

Thanks again,
Dan Martin



> -----Original Message-----
> From: Daniel Morissette [mailto:morissette at dmsolutions.ca]
> Sent: Tuesday, February 18, 2003 12:10 PM
> To: Martin, Daniel
> Cc: mapserver-users at lists.gis.umn.edu
> Subject: Re: [Mapserver-users] Can't seem to get the 
> projection correct
> 
> 
> "Martin, Daniel" wrote:
> > 
> > Sorry to correct you, but that's not true - at least not 
> since I've worked
> > with MapInfo (version 5 through 7). That may be an optional 
> configuration,
> > but by default MapInfo doesn't change the aspect ratio of 
> the data with the
> > window.  By default, the data's horizontal extent stays the 
> same as the
> > window is resized, but the vertical changes to maintain the 
> projection.
> > 
> 
> You're correct, I was wrong on that, I should have checked first.  I
> found the option that's causing this effect... open your lat/lon map,
> then select Map/Options, the "Distance/Area Using:" radio box 
> is set to
> Spherical.  Change the projection to "Non-Earth", and the
> "Distance/Area" will switch to cartesian, and then your map will look
> exactly as it does in MapServer.  
> 
> I'm not sure what this spherical distance option does exactly but it
> seems that MapInfo (and MapQuest) use this to adjust the 
> x-scale of the
> map to make it look better, that's why the map looks 
> different, this is
> not a projection effect, it's a map scaling effect.  Unfortunately at
> this point MapServer supports only maps with x-scale == y-scale (or
> square pixels if you like) so you won't be able to reproduce 
> that unless
> you can define a projection that produces a similar effect.
> 
> -- 
> ------------------------------------------------------------
>  Daniel Morissette               morissette at dmsolutions.ca
>  DM Solutions Group              http://www.dmsolutions.ca/
> ------------------------------------------------------------
> 
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