[postgis-users] Trouble with ST_ShortestLine: the returned line DOES NOT start in g1 and end in g2

[Jorge Gustavo Rocha]

Hi,

Help needed :-) I'm having trouble with lines returned by 
ST_ShortestLine function.

The computed lines *should* start and end on the geometries [1], but it 
is not happening.

In fact, the computed geometry sometimes crosses the original geometries 
or does not touch the geometries at all.

EXAMPLE 1: (of a computed geometry between a point and a line, that does 
not touches the line)

with
p as (
	select st_geometryfromtext('SRID=3763;POINT (-29802.23305474065 
138518.7938969792)') as geom
),
l as (
	select st_geometryfromtext('SRID=3763;LINESTRING (-29796.696826656284 
138522.76848210802, -29804.3911369969 138519.3504205817)') as geom
),
short as (
	select ST_ShortestLine(l.geom, p.geom) as geom
	from l, p
)
select short.geom,
ST_Disjoint(short.geom, l.geom),
ST_Intersects(short.geom, l.geom),
ST_Touches(short.geom, l.geom),
ST_Crosses(short.geom, l.geom)
from short, l;

|geom 
                   |st_disjoint|st_intersects|st_touches|st_crosses|
|-----------------------------------------------------------------------------------------|-----------|-------------|----------|----------|
|LINESTRING (-29802.795222153436 138520.05937757515, -29802.23305474065 
138518.7938969792)|true       |false        |false     |false     |

EXAMPLE 2: (of a computed geometry between a point and a line. The 
computed line crosses the original line.)

l as (
	select st_geometryfromtext('SRID=3763;LINESTRING (-29790.144327955728 
138508.0183209069, -29798.28270998299 138504.40298837385)') as geom
),
short as (
	select ST_ShortestLine(l.geom, p.geom) as geom
	from l, p
)
select short.geom,
ST_Disjoint(short.geom, l.geom),
ST_Intersects(short.geom, l.geom),
ST_Touches(short.geom, l.geom),
ST_Crosses(short.geom, l.geom)
from short, l;

|geom 
                   |st_disjoint|st_intersects|st_touches|st_crosses|
|-----------------------------------------------------------------------------------------|-----------|-------------|----------|----------|
|LINESTRING (-29795.33635541747 138505.7118543719, -29794.774188004685 
138504.44637377595)|false      |true         |false     |true      |

In both cases, I was expecting that the computed line touch the original 
line.

Am I understanding the ST_ShortestLine incorrectly or should I report a 
possible error? Can this be an (math approximation) error?

I'm using Postgis 3.3 dev, GEOS 3.11

POSTGIS="3.3.0dev 3.1.0rc1-1150-g05d3150c9" [EXTENSION] PGSQL="140" 
GEOS="3.11.0dev-CAPI-1.16.0" PROJ="8.2.0" LIBXML="2.9.12" 
LIBPROTOBUF="1.3.3" WAGYU="0.5.0 (Internal)"


Thanks in advance,

Jorge Gustavo

[1] https://postgis.net/docs/ST_ShortestLine.html
-- 
Jorge Gustavo Rocha
Departamento de Informática
Universidade do Minho
4710-057 Braga
Gabinete 3.29 (Piso 3)
Tel: +351 253604480
Fax: +351 253604471
Móvel: +351 910333888
skype: nabocudnosor