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<p>Paul Ramsey wrote:

<blockquote TYPE=CITE>Interesting. Learn something new every day. For the

benefit of all, here

<br>is the ESRI WTK definition of NTF_Paris_France_I <27581>

<p>PROJCS[

<br>  "NTF_Paris_France_I",

<br>  GEOGCS["GCS_NTF_Paris",

<br>    DATUM["D_NTF",

<br>      SPHEROID["Clarke_1880_IGN",6378249.2,293.46602]

<br>    ],

<br>    PRIMEM["Paris",2.337229166666667],

<br>    UNIT["Grad",0.015707963267948967]

<br>  ],

<br>  PROJECTION["Lambert_Conformal_Conic"],

<br>  PARAMETER["False_Easting",600000],

<br>  PARAMETER["False_Northing",1200000],

<br>  PARAMETER["Central_Meridian",0],

<br>  PARAMETER["Standard_Parallel_1",55],

<br>  PARAMETER["Scale_Factor",0.999877341],

<br>  PARAMETER["Latitude_Of_Origin",55],

<br>  UNIT["Meter",1]

<br>]

<p>Here is the definition I got from OGR converting from WKT to Proj4:

<p>  +proj=lcc

<br>  +lat_1=55.000000000

<br>  +lat_2=0.000000000

<br>  +lat_0=55.000000000

<br>  +lon_0=0.000000000

<br>  +x_0=600000.000

<br>  +y_0=1200000.000

<br>  +a=6378249.200

<br>  +b=6356515.000

<br>  +units=m

<p>As noted, there is no flag that the angular units are gradians-paris,

so

<br>they'll be misinterpretted as degrees-greenwich.</blockquote>

    ...

<p>All longitude is from the geographic coordinate origin <b>which is not</b>

<br><b>explicitly defined.</b>  So it can just as well be relative

to the French origin.

<br>The later, proj, definitioin is not in error unless one assumes that

the

<br>Greenwich origin is used.

<p>To use Greenwich data with proj one can merge the 2.33 correction into

<br>the lon_0 factor.

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