[OpenLayers-Users] UTM scales mismatch with Max/MinScaleDenomination

[Andrea Aime]

Christopher Schmidt ha scritto:
> On Thu, Nov 29, 2007 at 04:35:13PM +0100, David E. Reksten wrote:
>> Hi,
>>
>> we're using GeoServer/TileCache/OpenLayers serving several datasets
>> stepwise by scale. We've set the sld (using Min/Maxscaledenominator)
>> to display a particular dataset from 1:1-50000 (meters), but it
>> doesn't display before around 1:39000.
>>
>> Our dataset is stored as UTM32, and at first I thought the above to be
>> related to this bug: http://trac.openlayers.org/ticket/958
>> But the error is still there after applying the patch manually.
>>
>> We used OpenLayers.Util.getResolutionFromScale(50000, 'm') to get the
>> contant used in tilecache.cfg and in OpenLayers.
> 
> /** 
>  * Constant: DOTS_PER_INCH
>  * {Integer} 72 (A sensible default)
>  */
> OpenLayers.DOTS_PER_INCH = 72;

Hi Chris,
about this scale inconsistency, you're right in saying it's due to the
DPI. The WMS standard suggests a DPI of approximately 90 (well, a 
multiplication factor of 0.28 in fact), and that's what GeoServer is 
using to compute the scale.
To be exactly true to the WMS standard (as far as I understand it) the 
setting would have to be:

OpenLayers.DOTS_PER_INCH = 25.4 / 0.28;

Wondering if OL should default to it too (or is MapServer defaulting to 
75dpi instead?).

Cheers
Andrea