[OSRS-PROJ] Re: Older Stateplane Definitions

[Paul Ramsey]

Frank,
How confident are you of the NAD27 stateplane definitions in the EPSG
file? They tend to be in feet, and I am not sure that the offsets are
right. For example, King County in Washington say that their data is in
stateplane in the Washington North zone (epsg:32048).  The EPSG file
says this:

+proj=lcc +lat_1=47.500000000 +lat_2=48.733333333 +lat_0=47.000000000
+lon_0=-120.833333333 +x_0=609601.219 +y_0=0.000 +ellps=clrk66
+datum=NAD27 +to_meter=+0.3048006096

But by trial and error I found eventually that the actual projection was
this:

+proj=lcc +lat_1=47.500000000 +lat_2=48.733333333 +lat_0=47.000000000
+lon_0=-120.833333333 +x_0=500000 +y_0=0.000 +ellps=GRS80 +datum=NAD83
+to_meter=+0.3048006096

Curiously, ESRI has a third definition:

PROJCS["NAD_1927_StatePlane_Washington_North_FIPS_4601",GEOGCS["GCS_North_American_1927",DATUM["D_North_American_1927",SPHEROID["Clarke_1866",6378206.4,294.9786982]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["False_Easting",2000000],PARAMETER["False_Northing",0],PARAMETER["Central_Meridian",-120.8333333333333],PARAMETER["Standard_Parallel_1",47.5],PARAMETER["Standard_Parallel_2",48.73333333333333],PARAMETER["Latitude_Of_Origin",47],UNIT["Foot_US",0.30480060960121924]]

Anyhow, one thing the ESRI and King County definitions have in common is
an integral value for the offset, which makes practical sense and makes
me doubt the correctness of the definition in the proj4 epsg file. It
looks like all the projections with feet as the unit have suspect
non-integral offsets. Perhaps the process which generated them should be
re-examined. I think dividing the offsets in the proj4 file out by the
to_meters value tends to give the "correct" integral value of the offset
in feet. Would a patch to the epsg file which makes this correction be
appreciated?

BTW, I have also extended the epsg file to include the "bonus" IDs which
ESRI defines. I generally just replace the default epsg file with my
extended version when I do a new proj4 install, but perhaps the
"enhanced" file could be included as part of the proj4 distro?

Frank Warmerdam wrote:
> 
> Marc,
> 
> It is my intention to incorporate it in a future epsg file released with
> PROJ.4.  I expect to build a library of "hand updates" and an easy way of
> keeping them integrated.
> 
-- 
      __
     /
     | Paul Ramsey
     | Refractions Research
     | Email: pramsey at refractions.net
     | Phone: (250) 885-0632
     \_
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