[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>
>>>
>>>PROJCS[
>>> "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
Paul,
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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