Good news on this. Further experimentation revealed that it is possible
to get reasonably good agreement between scales in OpenLayers and GeoServer
using the following approach:
- use OpenLayers NON-geodesic (nominal) scale calculation
- set OpenLayers DOTS_PER_INCH to match the GeoServer value of 90
This produced OL scales that were in good agreement with the GeoServer
values (close enough for government work, anyway 8^). For example, at a
map nominal scale of 541,679 GeoServer computed the scale of 545,979
This was greatly assisted by Andrea's suggestion of using WMS Decorations,
in conjunction with some enhancements to my WMS Viewer (
http://dr-jts.github.io/maptest/maptest.html ).
For those interested, the following was used to display the GeoServer scale
on each map image:
GetMap parameter
format_options=layout:scaleratio
WMS Decoration Layout file scaleratio.xml:
layout>
<decoration type="scaleratio" affinity="bottom,left" offset="36,6"/>
</layout>
On Tue, Feb 16, 2016 at 9:35 AM, Andrea Aime <andrea.a...@geo-solutions.it>
wrote:
> On Tue, Feb 16, 2016 at 6:10 PM, Martin Davis <mtncl...@gmail.com> wrote:
>
>> Thanks, Andrea. I suspected this might be the cause of the issue.
>>
>> Is there a way to view the scale computed for a map request? That might
>> let us correlate the client (geodetic) scale to "OGC scale", so we can
>> translate between the two.
>>
>
> You can use the decorations:
> http://docs.geoserver.org/stable/en/user/advanced/wmsdecoration.html
>
> A "correlation" might be possible in a very specific local area, but just
> so that we are on the same
> page, the math is wildly different, so you could have a rough linear
> correlation in a town,
> not sure you can have that in a larger area (something spanning degrees,
> especially
> at that latitude where the mercator scale factor starts to diverge)
>
> Cheers
> Andrea
>
> --
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