[postgis-users] Newbie questions: SRIDs, function return values

[Paul Ramsey]

The zillow data is in lat/lon, so use -s 4326.
lat/lon is not a planar projection, so the units returned are pretty
non-sensical: square degrees for area, linear degrees for distance.

Once you have your data correctly flagged with respect to its source
projection, you can transform it.

Add a new entry for a continental USA projection:

INSERT INTO SPATIAL_REF_SYS VALUES (1000000,'ME',1000000,'','+proj=aea
+lat_0=23 +lon_0=-96 +lat_1=29.5 +lat_2=45.5 +datum=NAD83
+ellps=GRS80');

Then use it to calculate area:

SELECT area(transform(the_geom),1000000) FROM neighborhoods WHERE name
= 'Ballard';


P

On Wed, May 14, 2008 at 3:13 PM, David Jantzen <david at culturemob.com> wrote:
> Hi All,
>
> I'm brand spanking new to the GIS world, so I've got some pretty dumb
> questions.  If the answer is "RTFM" that's fine, please just be specific
> about which documentation to look at.
>
> What I'm trying to do:
> 1) Properly load the Zillow neighborhood shapefiles into a PostGIS
> enabled database.
> 2) Write some test queries to learn about the dataset and PostGIS
> functionality.
> 3) Take a lat/long and convert it to the appropriate neighborhood.
>
> I've made some progress on #1, but I'm not sure I've done it properly
> with respect to the SRID.  I loaded the Washington state data with this
> command:
>
> shp2pgsql ZillowNeighborhoods-WA.shp neighborhoods | psql GeoTest -U
> postgres
>
> The data loads without error and geometry_columns now contains:
>
> f_table_name  | f_geometry_column |coord_dimension | srid |  type
> -----------------+----------------+---------------+-------------------+--
> neighborhoods | the_geom          |              2 |   -1 | MULTIPOLYGON
>
> I'm concerned that I have to specify the correct SRID, however I don't
> see it anywhere in the Zillow .prj files, so I'm letting it default to
> -1 hoping it will be magically correct.  I suspect this may be
> problematic.
>
> Next, I'm attempting some basic queries, such as:
> select area(the_geom) from neighborhoods where name = 'Ballard';
>         area
> ----------------------
>  0.000461824752619577
>
> select distance((select the_geom from neighborhoods where name =
> 'Ballard'),(select the_geom from neighborhoods where name =
> 'Georgetown'));
>      distance
> --------------------
>  0.0982236357558739
>
> My question is, what do these return values *mean*? I read in the FAQ
> that "ST_Distance always returns in the units of the projection." which
> I assume applies to other functions as well.  And my understanding is
> that the units of the projection is determined by the SRID, and if
> that's wrong then any conclusions derived from it would be wrong as
> well.  Right?  So I'm kind of stuck here.  Is the -1 SRID really a
> problem?  How do I find the real SRID if it is?
>
> Thanks for your help,
> David
>
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