[postgis-users] Different srtext for the same SRID ?

Frank Warmerdam warmerdam at pobox.com
Mon Feb 27 06:49:37 PST 2006


Arnaud Lesauvage wrote:
> Hi list !
> 
> I have received some shapefiles containing data in UTM31N 
> coordinates-system (SRID=32631).
> The data came with a .prj file (one for each set of .shp and .dbf) 
> containing a PROJCS string.
> This string differs in some ways with the one in my spatial_ref_sys 
> table, and I was wondering whether I had to worry about that or not.
> 
> Here is the one from my dataset :
> PROJCS["UTM-31N",
> GEOGCS["WGS84",
> DATUM["WGS84",SPHEROID["WGS84",6378137.000000,298.257224]],
> PRIMEM["Greenwich",0.0],
> UNIT["Degree",0.0174532925199433]],
> PROJECTION["Transverse_Mercator"],
> PARAMETER["False_Easting",500000.0000000000],
> PARAMETER["False_Northing",0.0000000000],
> PARAMETER["Central_Meridian",3.0000000000],
> PARAMETER["Latitude_Of_Origin",0.0000000000],
> PARAMETER["K0",0.999600],
> UNIT["Non-Meter",1.000000]]
> 
> And the one from PostGIS (I removed the "AUTHORITY" strings for better 
> readability) :
> PROJCS["WGS 84 / UTM zone 31N",
> GEOGCS["WGS 84",
> DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563]],
> PRIMEM["Greenwich",0],
> UNIT["degree",0.01745329251994328]],
> PROJECTION["Transverse_Mercator"],
> PARAMETER["latitude_of_origin",0],
> PARAMETER["central_meridian",3],
> PARAMETER["scale_factor",0.9996],
> PARAMETER["false_easting",500000],
> PARAMETER["false_northing",0],
> UNIT["metre",1]]
> 
> There are two ovious differences : the PostGIS version has a scale 
> factor whereas the .prf file has a "KO" property with the same value. I 
> don't know what this means, but does it matter ?
> Second difference is that the .prj file says that the unit is 
> "non-meter", whereas PostGIS says that it is "metre" (="meter" in French).
> 
> Does any of this difference have some kind of importance ?

Arnaud,

These are clearly the same coordinate system.  I have never seen anyone
generate WKT with a "K0" parameter instead of "scale_factor", and if you
can find out I would be interested to know what software this came from.
In other regards, the WKT is OK if quirky.  Names for stuff like units are
just for understandability of the human, so calling a meter "Non-meter" is
harmless though confusing.

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




More information about the postgis-users mailing list