Generating line offset, perpendicular using PostGIS?

Question

I'm trying to generate two line offsets from a line but I'm having problems generating them as they should be.

Using PostGIS

select st_asgeojson(st_union(the_geom)) from ( select ST_Line_Substring(ST_OffsetCurve(line_geom,0.00005),0,1) as the_geom from table_name where id = 7382 UNION select ST_Line_Substring(ST_OffsetCurve(line_geom,-0.00005),0,1) as the_geom from table_name where id = 7382 UNION select (line_geom) as the_geom from table_name where id = 7382) as a

the problem I'm seeing right now is the angle between the start/end of given line isn't perpendicular to the start/end of the other lines

Result: Expected:

EDIT: lines' geometry and srid

"LINESTRING(-8.69371814905576 40.5676685897826,-8.69334901004008 40.567864446704)",4326 "LINESTRING(-8.6936947144568 40.5676244216994,-8.69332557544112 40.5678202786208)",4326 "LINESTRING(-8.69330214084216 40.5677761105376,-8.69367127985784 40.5675802536163)",4326

Answer 1

It was certainly a projection issue. You should first make sure you project your data into a local system. Here is what I used (for an offset of 2 meter in the portugese projection 3763):

WITH line AS (
SELECT 
ST_Transform(
    ST_SetSrid(
        ST_GeomFromText('LINESTRING(-8.6936947144568 40.5676244216994,-8.69332557544112 40.5678202786208)')
    ,4326) 
,3763)
geom
)
SELECT ST_OffsetCurve(geom,2) geom FROM line
UNION ALL
SELECT ST_OffsetCurve(geom,-2) geom FROM line
UNION ALL
SELECT geom FROM line;

I removed your LineSubstring since it didn't do anything.