[postgis-users] Using SRID with meters, but still not getting UNITS in meters with ST_DWithin

[Lucas Fairchild-Madar]

You can cast to geography only within your ST_Distance call to get meters:

ST_Distance(t1.geo_position::geography,t2.geo_position::geography)

That way the rest of your query will still use the index.

On Mon, Apr 11, 2016 at 11:21 AM, Michael Moore <michaeljmoore at gmail.com>
wrote:

> I am trying to find all zip codes withing a given range of current zip
> code. For example, User is at zip code 95076 and want to know all zip codes
> within a 30 mile range. This will always be a short distance, nothing over
> 50 miles. The source zip code is any place in the USA.  My input table has
> a latitude field, a longitude field and a POINT field for each zip code.
> I want to supply in input in meters, not degrees. I do NOT want to cast to
> *geography* because this prevents the index from being used.
> Here is a *working query* that uses the index, but ST_DWithin  is not
> understanding ".05" as meters:
>
>
> lcd1_dev=>   select t2.city, t2.postalcode,
> ST_Distance(t1.geo_position,t2.geo_position)
> lcd1_dev->   from      tpostalcoordinate t1
> lcd1_dev->   left join tpostalcoordinate t2 on ST_DWithin(t1.
> *geo_position*,t2.*geo_position* , .05)
> lcd1_dev->   where t1.postalcode = '94404'and t1.countrycode2tcountry =
> 'US' and t2.countrycode2tcountry= 'US';
>      city     | postalcode |     st_distance
> --------------+------------+---------------------
>  Redwood City | 94065      |  0.0273766323714193
>  San Mateo    | 94408      | 0.00504738546179631
>  Belmont      | 94002      |  0.0440065904155286
>  San Mateo    | 94404      |                   0
>  San Mateo    | 94403      |  0.0370314731005901
>  San Mateo    | 94407      |  0.0416118372581607
>
> *This shows that I am using SRID 4896*
> lcd1_dev=> select * from geometry_columns where f_table_name =
> 'tpostalcoordinate';
>  f_table_catalog | f_table_schema |   f_table_name    | f_geometry_column
> | coord_dimension | srid | type
>
> -----------------+----------------+-------------------+-------------------+-----------------+------+-------
>  lcd1_dev        | qsn_app        | tpostalcoordinate | geo_position
>  |               2 | 4896 | POINT
>
> *4896 is UNIT "metre" as show here:*
>  4896 | EPSG      |      4896 |
> GEOCCS["ITRF2005",DATUM["International_Terrestrial_Reference_Frame_2005",SPHEROID["GRS
> 1980",6378137,298.257222101,AUTHORIT
>
> Y["EPSG","7019"]],AUTHORITY["EPSG","6896"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
> UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Geocentric X",OTH
> ER],AXIS["Geocentric Y",OTHER],AXIS["Geocentric
> Z",NORTH],AUTHORITY["EPSG","4896"]] | +proj=geocent +ellps=GRS80 +units=m
> +no_de
>
>
> *The following (from EXPLAIN) proves that the index is being used in the
> working query shown above:*   ->  Index Scan using tpostalcoordinate_pk
> on tpostalcoordinate t1  (cost=0.42..8.45 rows=1 width=32)
>
> What do I need to do to get this in meters without losing my index access?
>
> TIA,
> Mike
>
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