[postgis-users] srid
Bob Pawley
rjpawley at shaw.ca
Mon May 26 12:06:23 PDT 2008
Hi Chris
I need to have a closer look at what you sent.
However, I attempting to interpret functions made for geographic data to use
with geometric data.
I have PostgreSQL tables which represent engineering processes.
I want to display that data in a graphical form - hopefully using
Postgis - - - if I can translate the functions (or the Postgis concept)
into a form that I can use.
Bob
----- Original Message -----
From: "Chris Hermansen" <chris.hermansen at timberline.ca>
To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
Sent: Monday, May 26, 2008 11:47 AM
Subject: Re: [postgis-users] srid
> Bob, maybe it's just me, but your questions about srid seem to be
> exploring a strange part of that structure.
>
> For example, I have a student project database here called "project1"
> (nice name, eh?). It contains a bunch of geographic tables; for
> example, the table "sitios".
>
> "sitios" looks like this:
>
> Table "public.sitios"
> Column | Type |
> Modifiers
> ------------+-------------------+------------------------------------------------------
> gid | integer | not null default
> nextval('sitios_gid_seq'::regclass)
> codigo | character varying |
> sector | character varying |
> nombreluga | character varying |
> funcion | character varying |
> comentario | character varying |
> the_geom | geometry |
> Indexes:
> "sitios_pkey" PRIMARY KEY, btree (gid)
> "sitios_geo_idx" gist (the_geom)
> "sitios_idx" btree (gid)
> Check constraints:
> "enforce_dims_the_geom" CHECK (ndims(the_geom) = 2)
> "enforce_geotype_the_geom" CHECK (geometrytype(the_geom) =
> 'POINT'::text OR the_geom IS NULL)
> "enforce_srid_the_geom" CHECK (srid(the_geom) = 24879)
>
>
> Note that the geometry is in a column called "the_geom".
>
> If I look in the table "geometry_columns", I see the following row
> corresponding to "sitios":
>
> project1=# select * from geometry_columns where f_table_name =
> 'sitios';
> f_table_catalog | f_table_schema | f_table_name | f_geometry_column
> | coord_dimension | srid | type
> -----------------+----------------+--------------+-------------------+-----------------+-------+-------
> | public | sitios | the_geom
> | 2 | 24879 | POINT
>
>
> See the attribute "srid" in table "geometry_columns"? For table
> "sitios", it's set to 24879. I can find this srid in the table
> "spatial_ref_sys" as follows:
>
> project1=# select * from spatial_ref_sys where auth_srid = 24879;
> srid | auth_name | auth_srid
> |
> srtext
> | proj4text
> -------+-----------+-----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------
> 24879 | EPSG | 24879 | PROJCS["PSAD56 / UTM zone
>
> 19S",GEOGCS["PSAD56",DATUM["Provisional_South_American_Datum_1956",SPHEROID["International
>
> 1924",6378388,297,AUTHORITY["EPSG","7022"]],AUTHORITY["EPSG","6248"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4248"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-69],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",10000000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AUTHORITY["EPSG","24879"]]
> | +proj=utm +zone=19 +south +ellps=intl +units=m +no_defs
> (1 row)
>
> OK, so now I see that the geometry in table "sitios" is "point" (from
> "geometry_columns") and in srid 24879 which is PSAD56 - provisional
> south american datum 56 (from "spatial_ref_sys").
>
> Lastly, if I look at the geometry itself in "sitios":
>
> project1=# select asText(the_geom) from sitios;
> astext
> ------------------------------------------
> POINT(741614.062489468 5628072.4999648)
> POINT(674306.187446182 5704431.49999243)
> POINT(719626.3749895 5828010.50002511)
> POINT(721600.874965506 5816697.50005473)
> POINT(707558.874971081 5819902.50002712)
>
> Now it seems that PostGIS "supports" embedded srids in the geometry, so
> you can also do the following:
>
> project1=# select asEWKT(the_geom) from sitios;
> asewkt
> -----------------------------------------------------
> SRID=24879;POINT(741614.062489468 5628072.4999648)
> SRID=24879;POINT(674306.187446182 5704431.49999243)
> SRID=24879;POINT(719626.3749895 5828010.50002511)
> SRID=24879;POINT(721600.874965506 5816697.50005473)
> SRID=24879;POINT(707558.874971081 5819902.50002712)
>
> In theory, I guess, this means you could have a separate / distinct SRID
> for each row of geometry, though I have to say I've never tried.
>
> But it sounds to me as though you want to have a different SRID for each
> row of your geometry, which doesn't sound like "the right thing to do"
> to me.
>
> OK, if you're still reading, does the above ring any bells for you? If
> not, explain to us what exactly you're trying to do with your use of
> st_transform() and maybe we can make some suggestions.
>
> Bob Pawley wrote:
>> I think I have the spatial_ref_table with a couple of rows that should
>> be recognized by the ST_Transform function.
>>
>> However, I now get the error "Input geometry has unknown (-1) srid".
>>
>> Does anyone have a thought on how to overcome this? Is this error
>> realing telling me that the function still doesn't recognize the srid?
>>
>> Bob
>>
>>
>>
>>
>> ----- Original Message ----- From: "Andy Anderson"
>> <aanderson at amherst.edu>
>> To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
>> Sent: Sunday, May 25, 2008 11:20 AM
>> Subject: Re: [postgis-users] srid
>>
>>
>>> SRID is an index into the table of datums/projections/coordinate
>>> systems that PostGIS maintains; the commands to create that table
>>> are in the file spatial_ref_sys.sql that comes with the distribution.
>>>
>>> If you want to build your own transformation, you could start with
>>> one of the listed INSERT statements that matches the type of
>>> projection, and change the coordinates to match your interest. Then
>>> add it to the existing table with an SRID that isn't being used.
>>>
>>> By the way, the SRID is part of the EPSG standard
>>> <http://en.wikipedia.org/wiki/SRID
>>> >, so keep that in mind for future potential conflicts.
>>>
>>> -- Andy
>>>
>>> On May 25, 2008, at 12:21 PM, Bob Pawley wrote:
>>>
>>>> Hi
>>>>
>>>> I am attempting to learn Postgis to use as a geometric tool rather
>>>> than a geographic tool.
>>>>
>>>> The function ST_Transform(geometry, integer) seems to be
>>>> potentially quite useful.
>>>>
>>>> Apparently, the integer required is the new coordinates referenced
>>>> by the srid. Is there some way of building a spatail reference
>>>> table in which I can enter my coordinates that the ST_Transform
>>>> function will recognize. My first simplistic attempt (entering a
>>>> coordinate with an id) didn't work.
>>>>
>>>> Bob
>>>> _______________________________________________
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>>>
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>>
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>
>
> --
> Regards,
>
> Chris Hermansen mailto:chris.hermansen at timberline.ca
> tel+1.604.714.2878 · fax+1.604.733.0631 · mob+1.778.232.0644
> Timberline Natural Resource Group · http://www.timberline.ca
> 401 · 958 West 8th Avenue · Vancouver BC · Canada · V5Z 1E5
>
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