[OSRS-PROJ] Units used in Expressing Projections
Clifford J Mugnier
cjmce at lsu.edu
Mon Jul 29 13:50:34 EDT 2002
Gentlemen,
The French use the secant case for Lambert Conformal Conics in France and
Corsica. The use of a Latitude of Origin and a Scale Factor at Origin is
not a "Tangent case" unless the scale factor at origin is 1.0. The one
latitude they quote is NOT a standard parallel. Curiously, the definition
they use is the "British Method." The original "French Method" is what the
U.S. Coast and Geodetic Survey (now National Geodetic Survey) uses by
listing two standard parallels. The resultant projection is identical,
only the algebra is different in reaching the final Constant of the Cone.
The only weird stuff the French used to do with Lambert is the French Army
Truncated Cubic Lambert Conic. That has not been used since 1948 in
France. Still found occasionally in North Africa and the Palestine,
though.
Clifford J. Mugnier (cjmce at LSU.edu)
Chief of Geodesy
CENTER FOR GEOINFORMATICS
Department of Civil Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA 70803
Voice and Facsimile: (225) 578-8536
Pager: 1-(888) 365-5180
================================
http://www.ASPRS.org/resources.html
http://www.ce.LSU.edu/~mugnier/
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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,
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