<div dir="ltr">Paul's comment about constructed points not necessarily lying on lines due to numerical rounding is 100% correct. However, in this particular case it turns out that the fact that ST_Intersects reports false is a bug in GEOS. See:<div><br></div><div><a href="https://github.com/libgeos/geos/issues/565">https://github.com/libgeos/geos/issues/565</a><br></div><div><br></div><div>The good news is that a fix has been identified and will be implemented in GEOS soon. </div><div><br></div><div>Handling robustness problems in computation geometry algorithms is a long-standing research problem. So any suggested solution should be made in that perspective.</div><div><br></div><div>My thinking is that a way to deal with this is to support a distance tolerance on spatial predicates. This will require a new algorithm for evaluating predicates, which hopefully can be worked on in the near/medium term. </div><div> </div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Feb 9, 2022 at 4:51 PM Jorge Gustavo Rocha <<a href="mailto:jgr@di.uminho.pt">jgr@di.uminho.pt</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hi Paul,<br>
<br>
Thanks for the feedback.<br>
<br>
I would expect, as a assertion:<br>
ST_Intersects( ST_ClosestPoint(line, point), line) = true<br>
<br>
In our current implementation, the ST_ClosestPoint(geometry g1, geometry <br>
g2) can return a point that is not on g1.<br>
<br>
If ST_ClosestPoint is returning a point that is not exactly on g1, due <br>
to the FP limitations, the ST_Intersects will have the same limitations <br>
and could agree that point is on the line.<br>
<br>
St_distance is reporting zero as the distance between the <br>
ST_ClosestPoint and the line, as expected. It seems like a exact zero.<br>
<br>
My point is, as a user, even knowing that the fp types are inexact, why <br>
st_distance is returning zero and st_intesects, st_touches, st_crosses, <br>
st_disjoint, are returning unexpected values?<br>
<br>
Maybe this can not be solved using the current fp representation, but <br>
maybe we can improve the logic to overcome some fp limitations.<br>
<br>
I'll push this issue to my Postgis stack of "todo".<br>
<br>
Regards,<br>
<br>
Jorge<br>
<br>
test code<br>
<br>
--8<-----------------<br>
<br>
--test code<br>
<br>
with<br>
p as (<br>
select st_geometryfromtext('SRID=3763;POINT (-29802.23305474065 <br>
138518.7938969792)') as geom<br>
),<br>
l as (<br>
select st_geometryfromtext('SRID=3763;LINESTRING (-29796.696826656284 <br>
138522.76848210802, -29804.3911369969 138519.3504205817)') as geom<br>
),<br>
short as (<br>
select ST_ShortestLine(l.geom, p.geom) as geom, ST_ClosestPoint(l.geom, <br>
p.geom) as closestpoint<br>
from l, p<br>
)<br>
select short.geom, short.closestpoint, ST_Intersects(short.closestpoint, <br>
l.geom), ST_StartPoint(short.geom),<br>
st_distance(short.closestpoint, l.geom) * 1E+24,<br>
ST_Disjoint(short.geom, l.geom),<br>
ST_Intersects(short.geom, l.geom),<br>
ST_Touches(short.geom, l.geom),<br>
ST_Crosses(short.geom, l.geom)<br>
from short, l;<br>
<br>
<br>
On 09/02/22 19:58, Paul Ramsey wrote:<br>
> If the shortest line runs vertext-to-vertext I would expect it to touch both lines. If on the other hand it runs vertex-to-midsegment I would expect it is possible it would not touch. Check the distance of the shortest line to the two parent lines. It should be zero or very very very small. That would be expected. It's not possible to place constructed points exactly on the lines they are constructed from, except in some cases and on vertices, thanks to the granularity of floating point representation. (Discrete math, not continuous math)<br>
> <br>
> P<br>
> <br>
>> On Feb 9, 2022, at 11:31 AM, Jorge Gustavo Rocha <<a href="mailto:jgr@di.uminho.pt" target="_blank">jgr@di.uminho.pt</a>> wrote:<br>
>><br>
>> Hi,<br>
>><br>
>> Help needed :-) I'm having trouble with lines returned by ST_ShortestLine function.<br>
>><br>
>> The computed lines *should* start and end on the geometries [1], but it is not happening.<br>
>><br>
>> In fact, the computed geometry sometimes crosses the original geometries or does not touch the geometries at all.<br>
>><br>
>> EXAMPLE 1: (of a computed geometry between a point and a line, that does not touches the line)<br>
>><br>
>> with<br>
>> p as (<br>
>> select st_geometryfromtext('SRID=3763;POINT (-29802.23305474065 138518.7938969792)') as geom<br>
>> ),<br>
>> l as (<br>
>> select st_geometryfromtext('SRID=3763;LINESTRING (-29796.696826656284 138522.76848210802, -29804.3911369969 138519.3504205817)') as geom<br>
>> ),<br>
>> short as (<br>
>> select ST_ShortestLine(l.geom, p.geom) as geom<br>
>> from l, p<br>
>> )<br>
>> select short.geom,<br>
>> ST_Disjoint(short.geom, l.geom),<br>
>> ST_Intersects(short.geom, l.geom),<br>
>> ST_Touches(short.geom, l.geom),<br>
>> ST_Crosses(short.geom, l.geom)<br>
>> from short, l;<br>
>><br>
>> |geom |st_disjoint|st_intersects|st_touches|st_crosses|<br>
>> |-----------------------------------------------------------------------------------------|-----------|-------------|----------|----------|<br>
>> |LINESTRING (-29802.795222153436 138520.05937757515, -29802.23305474065 138518.7938969792)|true |false |false |false |<br>
>><br>
>> EXAMPLE 2: (of a computed geometry between a point and a line. The computed line crosses the original line.)<br>
>><br>
>> l as (<br>
>> select st_geometryfromtext('SRID=3763;LINESTRING (-29790.144327955728 138508.0183209069, -29798.28270998299 138504.40298837385)') as geom<br>
>> ),<br>
>> short as (<br>
>> select ST_ShortestLine(l.geom, p.geom) as geom<br>
>> from l, p<br>
>> )<br>
>> select short.geom,<br>
>> ST_Disjoint(short.geom, l.geom),<br>
>> ST_Intersects(short.geom, l.geom),<br>
>> ST_Touches(short.geom, l.geom),<br>
>> ST_Crosses(short.geom, l.geom)<br>
>> from short, l;<br>
>><br>
>> |geom |st_disjoint|st_intersects|st_touches|st_crosses|<br>
>> |-----------------------------------------------------------------------------------------|-----------|-------------|----------|----------|<br>
>> |LINESTRING (-29795.33635541747 138505.7118543719, -29794.774188004685 138504.44637377595)|false |true |false |true |<br>
>><br>
>> In both cases, I was expecting that the computed line touch the original line.<br>
>><br>
>> Am I understanding the ST_ShortestLine incorrectly or should I report a possible error? Can this be an (math approximation) error?<br>
>><br>
>> I'm using Postgis 3.3 dev, GEOS 3.11<br>
>><br>
>> 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)"<br>
>><br>
>><br>
>> Thanks in advance,<br>
>><br>
>> Jorge Gustavo<br>
>><br>
>> [1] <a href="https://postgis.net/docs/ST_ShortestLine.html" rel="noreferrer" target="_blank">https://postgis.net/docs/ST_ShortestLine.html</a><br>
>> -- <br>
>> Jorge Gustavo Rocha<br>
>> Departamento de Informática<br>
>> Universidade do Minho<br>
>> 4710-057 Braga<br>
>> Gabinete 3.29 (Piso 3)<br>
>> Tel: +351 253604480<br>
>> Fax: +351 253604471<br>
>> Móvel: +351 910333888<br>
>> skype: nabocudnosor<br>
>> _______________________________________________<br>
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> <br>
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<br>
J. Gustavo<br>
-- <br>
Jorge Gustavo Rocha<br>
Departamento de Informática<br>
Universidade do Minho<br>
4710-057 Braga<br>
Gabinete 3.29 (Piso 3)<br>
Tel: +351 253604480<br>
Fax: +351 253604471<br>
Móvel: +351 910333888<br>
skype: nabocudnosor<br>
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</blockquote></div>