[Gdal-dev] gdalwarp parameters

[Frank Warmerdam]

Shoaib Burq wrote:
> Hi
> 
> I want to reproject an image which is currently in NAD83 Z21 to a
> custom projection.
> I am having some trouble getting the proj4 parameters to a custom
> projection right.
> I have been given the ArcGRID's prj.adf file and it looks like this:
> 
> 
> Projection    MERCATOR
> Datum         WGS84
> Spheroid      WGS84
> Units         METERS
> Zunits        NO
> Xshift        0.0
> Yshift        0.0
> Parameters
> -180  0  0.0 /* longitude of central meridian
>  45  0  0.0 /* latitude of true scale
> 0.0 /* false easting (meters)
> 0.0 /* false northing (meters)
> 
> Could some one please suggest the correct proj4 directive to give to
> gdalwarp's -t_srs ?
> 
> I have tried
>  "+proj=merc +lat_0=45.0 +lon_0=-180 +x_0=0 +y_0=0 +datum=WGS84
> +ellps=WGS84 +units=m"
> but this is not giving me the correct extents.

Shoaib,

I believe you should be using:

   +proj=merc +lat_ts=45 +lon_0=-180 +datum=WGS84

+lat_ts is "latitude of true scale"

Best regards,
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