Lining up offset problem WMS/Flash overlay

[Mark de Blois]

Hi all,

I am struggling with a slight offset in a new application we are 
developing which is a flash application (global risk mapping) and which 
will take in overlaid WMS layers served by MapServer in Winkel Triple. I 
have reprojected the base data (shapes, rasters) so that MapServer does 
not have to reproject on the fly, and performance seems to be good.

However, I am having a problem when zooming in and new WMS request are 
being made. The more you zoom in, the more offset becomes visible 
between the base (flash) layer (generated by exporting an ai file of the 
same shapefile) and the overlaid MapServer png.
Perhaps this has to do with the spatial extent of my data and the 
width/height ratio (SIZE)?! I used ogrinfo shape.shp -al -summary to 
generate the extent of my shape, which I then used as the extent of the 
map file. For the SIZE parameters I used the width I need for my png and 
calculated the height based on the width/height ratio in the above 
mentioned extent. Is this the right way to do so?! Or am I missing 
something here?

The image looks fine, except when zooming in and new requests are made 
using an adjusted bounding box (dimensions divided by a factor), the 
offset becomes slightly bigger with each zoom factor, as if a slight 
skewing is taking place.

Unlike the case in unprojected (lat/long) data, I cannot change these 
parameters much at all (about 2 pixels) before MapServer gives me an 
empty image. Is there a way in which MapServer will more flexibly allow 
me to change the range of these parameters? I also noticed that WMS 
requests give me bitmaps with dimensions consisting of whole integers. A 
rounding off takes place. This could result in some potential 
inaccuracies along borders too of course.

Some of these problems wouldn't occur if we were just using a MapServer 
based mapping application, however we do need to combine and line it up 
with the flash layer for other purposes.

I will now play around with the limited range in which I can change 
these parameters, but maybe I am missing a more obvious thing?!

Any help/advice on this issue would be greatly appreciated! I have 
included part of the map file below.

Regards,
Mark de Blois
Maplecroft

#
#Start of map file, created by AmeiN! for ArcGis 13/09/2006, 15:20:54
#
 
MAP
NAME 'MaplecroftWMS'
STATUS ON
 
PROJECTION
  "proj=wintri"
  "ellps=WGS84"
  "lat_1=45"
END
 
SIZE 697.8 401.31
EXTENT -16707480.310734 -10018754.171395 16920780.652974 9320947.177797
UNITS meters

LAYER
 NAME 'Countries_WT'
 DATA './admin/maplecroft_countries_winkel_45'
 STATUS off
 TYPE Polygon
TOLERANCE 7
 TOLERANCEUNITS pixels
 
 METADATA
  WMS_TITLE 'Countries_WT'
  WMS_FEATURE_INFO_MIME_TYPE 'text/html'
 END #METADATA
 
PROJECTION
  "proj=wintri"
  "ellps=WGS84"
  "lat_1=45"
END
 
 CLASS
  NAME 'Countries'
 
  STYLE
   #COLOR 255 255 212
   OUTLINECOLOR 255 0 0
  END #STYLE
 END #CLASS
 
TEMPLATE "winkel_query.html"
 
END #LAYER