[OpenLayers-Users] Questions: Projections, Google, Yahoo, etc

[Jon Blower]

Hi Olivier,

That's really useful, thanks very much.  Maybe metres are the
"accepted" unit but I still don't understand why.  The x-axis
(longitude) is non-linear in metres but linear in degrees.  This
surely means that it's harder for a client to construct a BBOX in
metres because the length of the bottom edge of the tile in (real)
metres is different from the length of the top edge of the tile in
metres.  Which length is used in the BBOX?  Or are the units really
arbitrary, just happening to coincide with metres at the equator?

I guess I can figure out how Geoserver works and then replicate this
behaviour in my WMS, but it looks very odd to me.

Am I alone in this or can others see the problem too?  Perhaps I've
misunderstood something.

Cheers, Jon


On 2/20/07, olivier.terral <olivier.terral at geomatys.fr> wrote:
> Hi,
>
> We use geoserver and mapbuidler (and OL in a  future for increase display time) for display NetCdf data
>
>
> In geoserver the definition of EPSG:41001 is this:
>
> PROJCS["WGS84 / Simple Mercator",
>   GEOGCS["WGS 84",
>     DATUM["WGS_1984",
>       SPHEROID["WGS_1984", 6378137.0, 298.257223563]],
>     PRIMEM["Greenwich", 0.0],
>     UNIT["degree", 0.017453292519943295],
>     AXIS["Lon", EAST],
>     AXIS["Lat", NORTH]],
>   PROJECTION["Mercator_1SP"],
>   PARAMETER["central_meridian", 0.0],
>   PARAMETER["scale_factor", 1.0],
>   PARAMETER["false_easting", 0.0],
>   PARAMETER["false_northing", 0.0],
>   UNIT["m", 1.0],
>   AXIS["x", EAST],
>   AXIS["y", NORTH],
>   AUTHORITY["EPSG","41001"]]
>
>
> So it's a projection in meters .
>
> We have display netCdf in mercator projection but problem for displaying
> data with  google map because it's in decimal degree.
> http://testbed.geomatys.fr/mapbuilder/demo/netCDF/
>
> I think problem can be resolve in OL because in Mapbuilder there are
> functions for calculates lat/lon in meters and inverse.
> I don't know exactly how OL proceed but when OL request a wms (with a
> google map base layer) Ol use epsg:4326, It must be possible to send a
> request in 41001 when the projection is in meters ( property: map.units)
>     and to use Mapbuilder functions for convert units ( may be creating
> a projection class in OL) .
>
> What do you think about ?
>
>
-- 
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Dr Jon Blower              Tel: +44 118 378 5213 (direct line)
Technical Director         Tel: +44 118 378 8741 (ESSC)
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ESSC                       Email: jdb at mail.nerc-essc.ac.uk
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