[OSRS-PROJ] Units used in Expressing Projections

Frank Warmerdam warmerdam at pobox.com
Mon Jul 29 13:13:32 EDT 2002

Paul wrote:
>>>Interesting. Learn something new every day. For the benefit of all, here
>>>is the ESRI WTK definition of NTF_Paris_France_I <27581>
>>>  "NTF_Paris_France_I",
>>>  GEOGCS["GCS_NTF_Paris",
>>>    DATUM["D_NTF",
>>>      SPHEROID["Clarke_1880_IGN",6378249.2,293.46602]
>>>    ],
>>>    PRIMEM["Paris",2.337229166666667],
>>>    UNIT["Grad",0.015707963267948967]
>>>  ],
>>>  PROJECTION["Lambert_Conformal_Conic"],
>>>  PARAMETER["False_Easting",600000],
>>>  PARAMETER["False_Northing",1200000],
>>>  PARAMETER["Central_Meridian",0],
>>>  PARAMETER["Standard_Parallel_1",55],
>>>  PARAMETER["Scale_Factor",0.999877341],
>>>  PARAMETER["Latitude_Of_Origin",55],
>>>  UNIT["Meter",1]
>>>Here is the definition I got from OGR converting from WKT to Proj4:
>>>  +proj=lcc
>>>  +lat_1=55.000000000
>>>  +lat_2=0.000000000
>>>  +lat_0=55.000000000
>>>  +lon_0=0.000000000
>>>  +x_0=600000.000
>>>  +y_0=1200000.000
>>>  +a=6378249.200
>>>  +b=6356515.000
>>>  +units=m


The code that morphs ESRI WKT definitions to "nominal" WKT in OGRSpatialReference
is wrong.  It maps the non-standard "Lambert_Conformal_Conic" projection name
to the Lambert_Conformal_Conic_2SP, and assumes a 0 value for the missing
Standard_Parallel_2.  It would appear that for ESRI WKT I need to inspect the
parameters and map to the 1SP or 2SP names depending on the parameters actually
available.  I will file this as a bug in bugzilla.

Gerald Evenden wrote:
> Using lat_1 and lat_2 specifies a secant version of LCC whereas the material
> in the WTK(?) version implies a tangent version with scale factor.  The later
> can
> be converted to a secant version but it certainly would not have 55 and 0
> secant
> cuts if the tangent versions point of tangency was at 55, the same as one of
> the
> secant cuts.
> Why not simplify the conversion and use the proj tangential form.

How would you express the projection in PROJ.4 syntax?  I am assuming
the correct tangental form would be:

+proj=lcc +lat_0=55 +lon_0=0 +lat_1=55 +k=0.999877341
    +x_0=600000 +y_0=1200000 +a=6378249.200 +b=6356515.000

Is that right?

Best regards,

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    | Geospatial Programmer for Rent

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