Sorry for the delayed response; I never got my message digest so I'm looking at the web archive to follow the discussion. Thanks for your help!
Paul: I added the continental US projection, and also had to add one for the 4326 projection. Is there more verbose documentation of these projections somewhere? I'm looking at spatial_ref_sys.sql and it's rather ... terse :) Andy: Thanks for the clarification. Bruce: I understand there's a huge conceptual foundation I'm missing here. Is there particular introductory material you would recommend? I should note that my goals are (I think) pretty modest. I need to map lat/longs into neighborhoods, and perform distance queries so that our users can restrict their search range to X number of miles from their homes. Thanks, David On Wed, 2008-05-14 at 15:13 -0700, David Jantzen 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 _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
