[Tilecache] Changing scales

[Oles Kit]

Dear list,

I seemed to be an easy trick, but I am playing with this problem for a
couple of days already without much success.

I've got a WMS server [MapServer] serving a map of the UK in
EPSG:27700 projection. This server is accessed by TileCache using the
following configuration:

[UK_27700]
type=WMS
url=http://......../mapfile.map
layers=uk
extension=png
size=256,256
srs=EPSG:27700
bbox=82445,3034,651251,396307
resolutions=496.34031413612564,248.17015706806282,124.08507853403141,62.042539267015705,31.021269633507853,15.510634816753926,7.755317408376963,3.8776587041884816,1.9388293520942408,0.9694146760471204,0.4847073380235602,0.2423536690117801,0.12117683450589005,0.060588417252945025,0.030294208626472512,0.015147104313236256

with OpenLayers javascript configured as:

map = new OpenLayers.Map('charts_map', {maxResolution: 'auto',
	maxExtent: new OpenLayers.Bounds(82445,3034,651251,396307),
	resolutions:
[496.34031413612564,248.17015706806282,124.08507853403141,62.042539267015705,31.021269633507853,15.510634816753926,7.755317408376963,3.8776587041884816,1.9388293520942408,0.9694146760471204,0.4847073380235602,0.2423536690117801,0.12117683450589005,0.060588417252945025,0.030294208626472512,0.015147104313236256], 


	projection: "EPSG:27700",
	units: "meters"});

This configuration performs well at the resolution of 496.34031413612564
meters per pixel and higher (i.e. when a 256-pixel tile roughly
represents a 127x127 km square). It seems logical to me that if I want
to reduce the scale of the map by the factor of 2 (i.e. to represent the
area of 254x254 km with one 256-pixel tile) I have to add one more
resolution to the list of resolutions, and namely 992.68062827225128,
but doing so I am getting unexpected results (usually wrong scale,
sometimes just a combination of shuffled tiles (puzzle-effect)). Playing
with bounding boxes, min/maxResolutions and Mapfile Capabilities does
not really help...

Any input is highly appreciated!

-- 
Oles Kit
GeoData Institute
University of Southampton