[Proj] Can I said the unit of Mercator Porjection? I'm trying to overlay shp file to google map.

[Frank Warmerdam]

baijianhua wrote:
 > I'm trying to overlay shape files(world map) to google map, I found that 
google
> map is using this PROJCS.
> 
> 54004,PROJCS["World_Mercator",GEOGCS["GCS_WGS_1984",DATUM["WGS_1984",SPHEROID
> ["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT
> ["Degree",0.017453292519943295]],PROJECTION["Mercator_1SP"],PARAMETER
> ["False_Easting",0],PARAMETER["False_Northing",0],PARAMETER
> ["Central_Meridian",0],PARAMETER["Standard_Parallel_1",0]
> 
> ,UNIT["Meter",1]
> 
> ,AUTHORITY["EPSG","54004"]]
> 
> 
> I can get the same shape of google map(in world range, the top most level of 
> google map), but one mismatch is the unit, the unit google map used is Decimal 
> Degree, while Mercator Projection using is Meter, when I change this to 
> 
> 54005,PROJCS["World_MercatorDD",GEOGCS["GCS_WGS_1984",DATUM["WGS_1984",SPHEROID
> ["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT
> ["Degree",0.017453292519943295]],PROJECTION["Mercator_1SP"],PARAMETER
> ["False_Easting",0],PARAMETER["False_Northing",0],PARAMETER
> ["Central_Meridian",0],PARAMETER["Standard_Parallel_1",0]
> 
> ,UNIT["Degree",0.0174532925199433]
> 
> ,AUTHORITY["EPSG","54005"]]
> 
> Still I can not get the same coordinate value as google map, can anybody help? 
> can I using the same unit with google map in Mercator projection result

Baijianhua,

It is not generally meaningful to use degrees as a linear unit for a
projected coordinate system in PROJ, OGR or simple features well known text
coordinate system descriptions.

Perhaps it is a property of mercator that a degree is linear in the
latitude direction (and equal number of mercator meters at any latitude),
and that it is the same distance in the longitude direction at any
latitude.   If so, feel free to put in a UNIT["degree",111111.0] or whatever 
the correct number of mercator meters for this pseudo-linear
unit is.

PS. I know I should know more about the properties of Mercator.  So shoot
me!

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org