[OSRS-PROJ] Units used in Expressing Projections
Gerald Evenden
gerald.evenden at verizon.net
Mon Jul 29 12:46:09 EDT 2002
Paul Ramsey wrote:
> This particular discussion started when I came across what appeared to
> be inconsistencies in the StatePlane definitions expressed in the proj4
> epsg file. As it turned out, even though the final units of those
> projections were feet, the offsets in the proj4 expression had to be in
> metres. In contrast, the ESRI WKT definitions gave the offsets in feet.
>
> While fixing up the proj4 definitions for some gradian-based projection
> expressions (like the one below), I noticed that the GEOGCS given in the
> ESRI WKT defines the PRIMEM for "Paris" as 2.3372, which is the angular
> distance from Paris to Greenwich in *DEGREES* even though the definition
> is in gradians. I guess the moral of the story is "know your units", or
> maybe it is "know your definitions", or perhaps "know your rules for
> expressing things".
>
> Has OGC completely abandoned WKT? I see work in the offing on an XML
> form of coordinate expression, but no explicit spec coming to iron out
> the many gaps in WKT which have been filled in by inference over the
> past several years.
>
> P.
>
> Paul Ramsey 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
> ----------------------------------------
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.
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