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

[Burgholzer,Robert]

Paul,
Nice example on that generic spatial ref system for calculating
distances and areas in the continental US.   This reminds me of a thread
about this a week or two ago.  It seems to me that the need exists (or
at lease it could be useful) for an area/distance functions that can
make some intelligent guesses about the best coordinate system to
reproject (transform) a given set of geometries into for the purposes of
accurate measurements.  Something like:

St_AreaUnits(geom_col, units)
St_distanceUnits(geom._col, units)

I am wondering if there is some logic out there that could be applied to
create a function that would evaluate the extent of the area and compare
it to the projection information described in the spatial_ref_sys table
to determine what srid (with the desired units) would have the greatest
chance of encompassing the area of interest and then do the transform
and measurement for the user.

Any recommendations as to the functions to look at within the existing
postgis suite to find the gizmos that pull apart the spatial
descriptions to determine this? 

Or is this line of reasoning based on flawed assumptions?

Thanks!

Robert W. Burgholzer
Surface Water Modeler
Office of Water Supply and Planning
Virginia Department of Environmental Quality
rwburgholzer at deq.virginia.gov
804-698-4405
Open Source Modeling Tools:
http://sourceforge.net/projects/npsource/

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Paul
Ramsey
Sent: Wednesday, May 14, 2008 6:36 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Newbie questions: SRIDs, function return
values

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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