[OSRS-PROJ] Units used in Expressing Projections

Paul Ramsey pramsey at refractions.net
Mon Jul 29 11:54:03 EDT 2002

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.


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>
>   "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
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