Hi,
thx for explanation.
Regards,
Andrea.
Il 11/01/2011 21:53, Nicklas Avén ha scritto:
Hallo Andrea
The two functions is very different but behave the same way in a few
specific situations.
St_Intersection I guess you know how works.
ST_Closestpoint is actually a distance function. It is the forst point
of ST_Shortestline. The pupose of those functions is to use on
geometries not intersecting at all. Look at the illustrations in the
documentation:
http://www.postgis.org/documentation/manual-1.5/ST_ShortestLine.html
ST_Shortestline gives you the line along which ST_Distance has found the
min distance between two geometries. If you use the shortestline between
a lake and a road it will give you the shortest possible line between
the lake and the road.
And as said, ST_Closestpoint is the first point in the shortest line,
so:
ST_Startpoint(ST_Shortestline(geometry, geometry)) =
ST_Closestpoint(geometry, geometry)
St_closestpoint is there because we expected it to be the most common
usecase of ST_Shortestline to just find where on my geometry is the
closest point to that geometry over there.
That it gives you the same answer when using it with a point
intersecting with a polygon is just a way to handle that situation. The
alternative is to return null. SQL Server is comming up with a
corresponding function to ST_Shortestline in their next release, but
they call it ShortestLineTo. They have choosen to return null in case of
intersection, while postgis returns a zero-length line with the same
start and end in any of the intersecting points (if many)
HTH
Nicklas
On Tue, 2011-01-11 at 21:19 +0100, aperi2007 wrote:
Il 11/01/2011 21:12, Paolo Cavallini ha scritto:
Il giorno mar, 11/01/2011 alle 20.52 +0100, aperi2007 ha scritto:
Hi,
I need to find the intersection point between two lines,
the first choice is ST_Intersection, but I notice even the
ST_ClosestPoint function.
\df+ st_intersection
in the case of two lines, the two results converge, but if you have two
polygons, you can see the difference.
All the best.
Hi Paolo,
Of course this is a difference.
ST_Intersection is surely more flexible, usable with every kind of geometry.
But my question is: "why ST_ClosestPoint exist ?"
The only explain I can suppose is that for linear archives it give some
advantage (more speed ?)
This is the question :)
Andrea.
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