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

[Graham Davis]

This *might* be useful in gaining more information on how various 
projections work an why:
http://erg.usgs.gov/isb/pubs/MapProjections/projections.html

I've always known mercator as a "meters" projection, though I don't 
really know why that is either.

-- 
Graham Davis
Refractions Research Inc.
gdavis at refractions.net



Jon Blower wrote:

>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 ?
>>
>>
>>    
>>