direction - How can I calculate the bearing between two points in PostGIS?

How can I find the bearing between two points in PostGIS?

Please specify in your answer whether or not the method producing a bearing on the spheroid, or a planar bearing.

Answer

Using ST_Azimuth

Planar bearing can be calculated using ST_Azimuth:

SELECT ST_Azimuth(ST_MakePoint(1,2), 
           ST_MakePoint(3,4))/(2*pi())*360 as degAz,
       ST_Azimuth(ST_MakePoint(3,4), 
           ST_MakePoint(1,2))/(2*pi())*360 As degAzrev

degaz   degazrev

------  ---------
45      225

For spherical azimuth (Quoting potgis-users group):

The PostGIS azimuth function seems to use a simple arctan function to determine azimuth. If you convert your coordinates to a projected coordinate system and then run the query, your results will be much closer to the FCC site's results.
Here is a quick conversion to UTM Zone 31:

select degrees(azimuth(
'POINT(634714.442133176 5802006.052402816)',

'POINT(634731.2410598891 5801981.648284801)'
));

which yields an azimuth of 145.457858825445. Points in the center of the UTM zone, or a more suitable projection would give better results.

Using trigonometric functions and ST_distance_sphere

This is the solution I've chosen when I had to deal with this problems, mainly for legacy reasons (I had a Python function which calculates the azimuth). First, we need to find a function that would tell us the exact distance between two points. Quoting the postgis manual:

ST_distance_sphere(point, point) Returns linear distance in meters between two lat/lon points. Uses a spherical earth and radius of 6370986 meters. Faster than distance_spheroid(), but less accurate. Only implemented for points.

Measure the Longitude and Latitude distance between the points, and use the arctan function to retrieve the angle.