Reprojected ghost images

[Ed McNierney]

John -

The GB National Grid is a Transverse Mercator projection with a central
parallel of 49 degrees North.  A world map in that projection would look
rather strange, but should be possible (if inadvisable).  I realize you¹re
using a latitude/longitude output projection (equally inadvisable, but
that¹s another soapbox) but when you put your NG layer on that output map
you¹re in effect causing a world-sized view of your TM projection in order
to figure out where your TM source image should be placed.

I¹m not quite seeing how your particular symptom is produced by that
problem, however.  Is  your map viewer publicly accessible, and can you post
your map file?  Thanks.

    - Ed

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


From: John Westwood <John.Westwood at PORT.AC.UK>
Reply-To: John Westwood <John.Westwood at PORT.AC.UK>
Date: Fri, 27 Apr 2007 16:43:07 +0100
To: <MAPSERVER-USERS at LISTS.UMN.EDU>
Subject: [UMN_MAPSERVER-USERS] Reprojected ghost images

Hello fellow MapServer users,
 
I have a small problem with reprojecting a tileindexed raster layer. The
original layer is in GB National Grid coordinates and I have reprojected
this on the fly to latitude and longitude. The coverage of the layer extends
to the area of the United Kingdom. The re-projection works fine and I am
very happy with it.
 
The problem is if I zoom out to a whole World view, there are two duplicates
of the reprojected UK raster layer equally spaced on either side of the real
layer. The duplicates are on the same line of latitude as the real layer but
are approximately located in North America and East Russia. The duplicates
are also heavily distorted, and one is the mirror image of the other.
 
I would like to know what is causing this and how I can possibly prevent it.
I have a workaround which is to switch the layer off when I have zoomed out
a certain amount; this works ok at the moment but may cause problems in the
future. Any ideas would be greatly appreciated. I am using MapServer version
4.10.0-5+etch1 on Debian Etch.
 
Regards,
 
John W.


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