How do I convert a latitude/longitude pair into a PostGIS geography type?

Question

I\'m trying to load a bunch of latitude/longitude pairs into a PostGIS geography type so as to be able to query by location.

In particular I have a table with float lat

Answer 1

Here are some different ways to make geography types:

• Convert numeric long and lat columns to a geog geography type:

   UPDATE mytable SET geog = ST_SetSRID(ST_MakePoint(long, lat), 4326)::geography
  

• Convert a geom geometry column (SRID=4326) to a geog geography type using a simple cast:

   UPDATE mytable SET geog = geom::geography
  

• Transform a projected geom geometry column to a geog geography type:

   UPDATE mytable SET geog = ST_Transform(geom, 4326)::geography
  

Note that the last two examples work on any geometry type. Also, the conversion from geometry to geography is often implicit, and these examples work without ::geography, however explicit casts are usually a good practice for these things.

Answer 2

To perform exchange between lat and lng you may use:

update mytable set geography = ST_GeographyFromText('SRID=4326;POINT(' || st_x(geom) || ' ' ||  st_y(geom) || ')');

with or without srid.

Answer 3

...sigh. Stupidity on my part. Apparently the correct order is longitude, latitude. I was fooled into thinking that both coordinates had the same range (-180 to 180) so thought something more subtle was going on.