[gdal-dev] OGR/EPSG problem with California State Plane?

[Frank Warmerdam]

Ben Discoe wrote:
>>From: Ben Discoe [mailto:ben at v...]
>>Sent: Saturday, January 19, 2002 1:21 AM
>>To: gdal-dev at yahoogroups.com
>>Subject: [gdal-dev] OGR/EPSG problem with California State Plane?
>>
>>
>>I've spent the past day trying to track down a problem with some GIS data
>>i've acquired from the City of Palo Alto. They say that the data uses
>>California State Plane, Zone 3, NAD83.
>>
>>When i use OGRSpatialReference to operate on these coordinates, i get very
>>incorrect results.
>>
>>I've tracked down the problem to the set of parameters used to
>>describe the projection. They parameters are looked up by
>>EPSGGetProjTRFInfo() which consults the file
>>"gdal/data/trf_nonpolynomial.csv", and gets the following
>>data from the file:
>>
>>10433 California CS83 zone 3 [..] 36.3 -120.3 38.26 37.04 2000000 500000
>>
>>That is:
>>standard parallel 1 38.4333333333
>>standard parallel 2 37.0666666666
>>center latitude 36.5
>>center longitude -120.5
>>false easting 2000000
>>false northing 500000
>>
>>But, according to one online reference, the correct values are:
>>False Easting: 6561666.6666
>>False Northing: 1640416.6666

Ben,

The csv files are derived from the EPSG 4.3 database. While it possible that
it has errors, it is unlikely that there are alot on something as important
as state plane zones.

I notice the difference in easting and northing is exactly the factor of
the us foot vs. meter.

In other words, you are expressing the false easting and northing in us feet
while the database expresses them in meters. A dump in wkt of the database
for california NAD83 zone 3 shows:

PROJCS["NAD83 / California zone 3",
GEOGCS["NAD83",
DATUM["North_American_Datum_1983",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG",7019]],
TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG",6269]],
PRIMEM["Greenwich",0,
AUTHORITY["EPSG",8901]],
UNIT["degree",0.0174532925199433],
AUTHORITY["EPSG",4269]],
PROJECTION["Lambert_Conformal_Conic_2SP"],
PARAMETER["standard_parallel_1",38.4333333333333],
PARAMETER["standard_parallel_2",37.0666666666667],
PARAMETER["latitude_of_origin",36.5],
PARAMETER["central_meridian",-120.5],
PARAMETER["false_easting",2000000],
PARAMETER["false_northing",500000],
UNIT["metre",1,
AUTHORITY["EPSG",9001]],
AUTHORITY["EPSG",26943]]

Note that the overall units for zone are meters. As defined by law, California
3 zone 3 in NAD83 is always supposed to be operated on in meters. Of course, some
organizations and projects still operate in feet anyways. Perhaps your data does?

Note, that if you use the same WKT definition, but with the units being feet the
false northings and eastings still have to be expressed in meters.

Best regards,

-- 
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