How can I create a polygon using fields in PostgreSQL?

Question

I have 8 real values in a table that I'd like to combine into a polygon. I haven't been able to figure out how to create a polygon using these values though. I keep trying variations of

SELECT polygon(lat1,lon1,lat2,lon2,lat3,lon3,lat4,lon4) FROM table;

but keep getting errors about the polygon function not existing or an invalid input syntax for type polygon. Has anyone done this before?

Answer 1

The syntax for a regular postgres polygon is more like:

insert into geo_table values (1, '((2,2),(3,4),(3,6),(1,1))');

Where 1 is some id and the quoted entry is the polygon. I would expect the query to be similar, you probably need parentheses etc for the coordinates. Typically for geospatial data you want (Lon Lat) coordinates. Postgis also takes WKT statements like:

GeomFromText('POLYGON((long1 lat1, long2 lat2, long3 lat3))')

Answer 2

As mentioned by bvmou - GeomFromText will work fine. I'll just add a small syntax update:

GeomFromText('POLYGON((long1 lat1, long2 lat2, long3 lat3))')

Answer 3

This example takes longitude and latitude coordinates from a table and converts them into a geometry. The dimensions of each box are given as long_high, long_low, lat_high, and lat_low. Here, a box of approximately 500m by 500m.

1. Add a new geometry column 'box' to the table

   SELECT AddGeometryColumn('public', 'predpol_raw', 'box', 2240, 'POLYGON', 2);

2. Update the new field with those values.

   UPDATE predpol_raw
   SET box =
           ST_Transform(
               ST_GeomFromText(
                   format('POLYGON((%s %s, %s %s, %s %s, %s %s, %s %s))',
                       long_high,lat_high, long_low,lat_high,
                       long_low,lat_low, long_high,lat_low,
                       long_high,lat_high
                   ),
                   4326
               ),
               2240
           );
   

Note the transformation to a different spatial reference. The POLYGON keyword requires double parentheses '(( ))'.