[postgis-users] beginner questions re. SRID
pramsey at refractions.net
Fri Oct 21 07:03:41 PDT 2005
On Oct 21, 2005, at 6:37 AM, Rick Schumeyer wrote:
> Is there a more human-readable table that contains the SRIDs and
More than spatial_ref_sys? Not really. The SRTEXT isn't completely
> Are the SRIDs always the same? For example, SRID 2959 on my
> system is UTM zone 18 with the GRS80 ellipse. Is this always SRID
> 2959 for everybody?
No. (And Yes!) No, because the spatial_ref_sys table is internal to
your database, so the identifiers in the table (SRID) are internal to
the database. Yes, because the PostGIS spatial_ref_sys table uses an
externally defined number from the European Petroleum Survey Group
(EPSG) as the SRID. Note that spatial_ref_sys has an AUTH_NAME and
AUTH_SRID. Aha. These are the external authority and identifier.
For PostGIS they are the same as the internal identifier. For other
systems, not necessarily so. Clear as mud!
> The documentation includes this example:
> CREATE TABLE parks ( PARK_ID int4, PARK_NAME varchar(128),
> PARK_DATE date, PARK_TYPE varchar(2) );SELECT AddGeometryColumn
> ('parks', 'park_geom', 128, 'MULTIPOLYGON', 2 ); This column has an
> SRID of 128? Does this mean that only objects with SRID=128 can be
> inserted in the column? I thought you included an SRID when you
> INSERT an object?
Yes! Confusing! But part of the standard. The specification says
that all geometries have an SRID. It also says that the
geometry_columns table contains a single SRID for each spatial column.
> How does postgis use the SRID internally?
Only really for the transform(geometry,id) function, which takes the
SRID of the geometry as the input projection and converts to the
output projection defined by the id.
> Can it properly compare an object with one SRID with an object with
> a different SRID?
No, and most/many functions will error out if you try, because it is
not really correct to compare geometries with different SRID (unless
they happen to map to the same projection! ha!).
> Finally, if I have missed some documentation that explains this,
> please point me in that direction!
If you have learned something from this post, please distill it into
the wiki for others!
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