Beginner question: Incorrect SRID?

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Beginner question: Incorrect SRID?

Adrian Holovaty
Hello,

I've successfully imported the City of Chicago's ZIP code data into
PostgreSQL/PostGIS, but when I try to transform() it in Postgres, I get
longitude and latitude that appears to be way off. I suspect I'm importing it
using the wrong SRID, but I'm not sure. I'm a beginner and would appreciate
some help.

The Chicago GIS Web site from which I got the data (http://tinyurl.com/7qwad)
says it follows the following standards:

   Datum: NAD 83
   Projection: State Plane Illinois East
   FIPS Zone: 1201
   Units: US Foot
   Precision: 1:10,000   (1:1,000 is acceptable for most uses)

I've used shp2pgsql to import the data with SRID 26971, which looks correct to
my inexperienced eyes:

mapdatabase=# select * from spatial_ref_sys where srid=26971;
 26971 | EPSG      |     26971 | PROJCS["NAD83 / Illinois East",
GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS
1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4269"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",36.66666666666666],PARAMETER["central_meridian",-88.33333333333333],PARAMETER["scale_factor",0.999975],PARAMETER["false_easting",300000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","26971"]]                                      
| +proj=tmerc +lat_0=36.66666666666666 +lon_0=-88.33333333333333 +k=0.999975
+x_0=300000 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs

Here's the import command I used:

shp2pgsql -s 26971 Zipcodes.shp testtable > zipcodes.sql

Is this the correct SRID for this data? Do I have to do something special
because the data standard uses "US Foot" as its units?

Raw data, in shapefile format, is here:
http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/zipcodes.zip

The query I'm running on the data to get longitude/latitude is:
select zip, asText(transform(the_geom, 4326)) from testtable limit 1;

That query returns:
MULTIPOLYGON(((-75.3136373094386 53.5032244406271,-75.3116700756851
53.5029863825251 ...

...which corresponds to longitude and latitude somewhere in Canada instead of
Chicago.

Thanks for any help.

Adrian
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Re: Beginner question: Incorrect SRID?

Paul Ramsey
No, it's the wrong srid. Try 102671, which is the same thing in feet.  
That's the "made up" ESRI id for that projection. I think there's an  
official EPSG id now, but it isn't in our table...

Paul

On 29-May-05, at 12:58 PM, Adrian Holovaty wrote:

> Hello,
>
> I've successfully imported the City of Chicago's ZIP code data into
> PostgreSQL/PostGIS, but when I try to transform() it in Postgres, I  
> get
> longitude and latitude that appears to be way off. I suspect I'm  
> importing it
> using the wrong SRID, but I'm not sure. I'm a beginner and would  
> appreciate
> some help.
>
> The Chicago GIS Web site from which I got the data (http://
> tinyurl.com/7qwad)
> says it follows the following standards:
>
>    Datum: NAD 83
>    Projection: State Plane Illinois East
>    FIPS Zone: 1201
>    Units: US Foot
>    Precision: 1:10,000   (1:1,000 is acceptable for most uses)
>
> I've used shp2pgsql to import the data with SRID 26971, which looks  
> correct to
> my inexperienced eyes:
>
> mapdatabase=# select * from spatial_ref_sys where srid=26971;
>  26971 | EPSG      |     26971 | PROJCS["NAD83 / Illinois East",
> GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS
> 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY
> ["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT
> ["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY
> ["EPSG","4269"]],PROJECTION["Transverse_Mercator"],PARAMETER
> ["latitude_of_origin",36.66666666666666],PARAMETER
> ["central_meridian",-88.33333333333333],PARAMETER["scale_factor",
> 0.999975],PARAMETER["false_easting",300000],PARAMETER
> ["false_northing",0],UNIT["metre",1,AUTHORITY
> ["EPSG","9001"]],AUTHORITY["EPSG","26971"]]
> | +proj=tmerc +lat_0=36.66666666666666 +lon_0=-88.33333333333333  
> +k=0.999975
> +x_0=300000 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs
>
> Here's the import command I used:
>
> shp2pgsql -s 26971 Zipcodes.shp testtable > zipcodes.sql
>
> Is this the correct SRID for this data? Do I have to do something  
> special
> because the data standard uses "US Foot" as its units?
>
> Raw data, in shapefile format, is here:
> http://egov.cityofchicago.org/webportal/COCWebPortal/COC_ATTACH/ 
> zipcodes.zip
>
> The query I'm running on the data to get longitude/latitude is:
> select zip, asText(transform(the_geom, 4326)) from testtable limit 1;
>
> That query returns:
> MULTIPOLYGON(((-75.3136373094386 53.5032244406271,-75.3116700756851
> 53.5029863825251 ...
>
> ...which corresponds to longitude and latitude somewhere in Canada  
> instead of
> Chicago.
>
> Thanks for any help.
>
> Adrian
> _______________________________________________
> postgis-users mailing list
> [hidden email]
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>

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Re: Beginner question: Incorrect SRID?

Adrian Holovaty
Paul Ramsey wrote:
> No, it's the wrong srid. Try 102671, which is the same thing in feet.
> That's the "made up" ESRI id for that projection. I think there's an
> official EPSG id now, but it isn't in our table...

Thanks for the response. That SRID isn't available for me, though:

"""
$ shp2pgsql -s 102671 Zipcodes.shp testtable | psql testdb
$ psql testdb
testdb=# select zip, asText(transform(the_geom, 4326)) from testtable limit 1;
ERROR:  Cannot find input SRID (102671) in spatial_ref_sys
"""

Are you saying I should create it by adding a record to the spatial_ref_sys
table?

I'm using PostGIS 1.0.1, if that matters. Thanks again!

Adrian
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