[OSRS-PROJ] Re: Older Stateplane Definitions

[Frank Warmerdam]

Paul Ramsey wrote:
 > 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

Paul,

Note that what you found by trial and error is EPSG 32148 - Washington North
NAD83!  I think the first problem is just that the data is in the NAD83
stateplane zone coordinates, not the NAD27 coordinates.

Second, the +x_0 and +y_0 false eastings and northings are always in meters
regardless of the +to_meter or +units settings.  I verified this as follows:

warmerda at gdal[624]% cs2cs +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 +to +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 +units=m
0 0
-0.00   -0.00 0.000
100 100
30.48   30.48 0.000

As you can see both coordinate systems have the same origin but one is
feet and one in meters.

 > 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_M-
 > eridian",-120.8333333333333],PARAMETER["Standard_Parallel_1",47.5],PARAMETER["Standard_-
 > Parallel_2",48.73333333333333],PARAMETER["Latitude_Of_Origin",47],UNIT["Foot_US",0.3048-
 > 0060960121924]]
 > 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?

The ESRI WKT format has the false easting and northing in the same units as the
projected coordinate system, which is why they are integral (numbers of feet).
Note that the OGC Coordinate Transforms specification states that these values
are always in meters (unless otherwise indicated by a units field), so it is not
very easy to have different uses of WKT coordinate system interoperate sometimes. My
own OGRSpatialReference.morphFromESRI() method tries to correct for different
interpretations of false easting and northing.

In short, I think the definitions of the NAD27 state plane zones are correct in the epsg
file.

 > 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?

I would likely prefer to offer them as an "esri" lookup file along side the epsg file
rather than include the "bonus" ones right in the epsg file since they don't really
have anything to do with epsg.

I really need to update this stuff some time soon!

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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