[postgis-tickets] [SCM] PostGIS branch master updated. 3.2.0-581-g9cf58181c
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- Log -----------------------------------------------------------------
commit 9cf58181c69f961edd6976594bbecae55d8f6e80
Author: Martin Davis <mtnclimb at gmail.com>
Date: Fri Feb 18 11:24:49 2022 -0800
Improve doc for ST_HausdorffDistance
diff --git a/doc/html/image_src/Makefile.in b/doc/html/image_src/Makefile.in
index 8d0fe9dc3..6037b2522 100644
--- a/doc/html/image_src/Makefile.in
+++ b/doc/html/image_src/Makefile.in
@@ -75,6 +75,7 @@ IMAGES= \
../images/st_extrude03.png \
../images/st_generatepoints01.png \
../images/st_geometricmedian01.png \
+ ../images/st_hausdorffdistance01.png \
../images/st_issimple01.png \
../images/st_issimple02.png \
../images/st_issimple03.png \
diff --git a/doc/html/image_src/st_hausdorffdistance01.wkt b/doc/html/image_src/st_hausdorffdistance01.wkt
new file mode 100644
index 000000000..c41352d64
--- /dev/null
+++ b/doc/html/image_src/st_hausdorffdistance01.wkt
@@ -0,0 +1,4 @@
+ArgA;LINESTRING (20 70, 70 60, 110 70, 170 70)
+ArgB;LINESTRING (20 90, 130 90, 60 100, 190 100)
+Result;LINESTRING (52.69230769230769 63.46153846153846, 60 100)
+Highlight;LINESTRING (20 70, 20 90)
diff --git a/doc/reference_measure.xml b/doc/reference_measure.xml
index 254c80d03..577115939 100644
--- a/doc/reference_measure.xml
+++ b/doc/reference_measure.xml
@@ -890,26 +890,28 @@ The smaller densifyFrac we specify, the more acurate Fréchet distance we get. B
<refsection>
<title>Description</title>
- <para>Returns the Hausdorff distance between two geometries, a measure of how similar or dissimilar 2 geometries are.
+ <para>Returns the
+ <ulink url="http://en.wikipedia.org/wiki/Hausdorff_distance">Hausdorff distance</ulink>
+ between two geometries.
+ The Hausdorff distance is a measure of how similar or dissimilar 2 geometries are.
</para>
- <para>Implements algorithm for computing a distance metric which can be thought of as the "Discrete Hausdorff Distance".
-This is the Hausdorff distance restricted to discrete points for one of the geometries. <ulink url="http://en.wikipedia.org/wiki/Hausdorff_distance">Wikipedia article on Hausdorff distance</ulink>
- <ulink url="http://lin-ear-th-inking.blogspot.com/2009/01/computing-geometric-similarity.html">Martin Davis note on how Hausdorff Distance calculation was used to prove correctness of the CascadePolygonUnion approach.</ulink></para>
- <para>
-When densifyFrac is specified, this function performs a segment densification before computing the discrete hausdorff distance. The densifyFrac parameter sets the fraction by which to densify each segment. Each segment will be split into a number of equal-length subsegments, whose fraction of the total length is closest to the given fraction.
+ <para>The function actually computes the "Discrete Hausdorff Distance".
+ This is the Hausdorff distance computed at discrete points on the geometries.
+ The <parameter>densifyFrac</parameter> parameter can be specified,
+ to provide a more accurate answer by densifying
+ segments before computing the discrete Hausdorff distance.
+ Each segment is split into a number of equal-length subsegments
+ whose fraction of the segment length is closest to the given fraction.
</para>
<para>Units are in the units of the spatial reference system of the geometries.
</para>
<note>
<para>
-The current implementation supports only vertices as the discrete locations. This could be extended to allow an arbitrary density of points to be used.
- </para>
- </note>
- <note>
- <para>
- This algorithm is NOT equivalent to the standard Hausdorff distance. However, it computes an approximation that is correct for a large subset of useful cases.
- One important part of this subset is Linestrings that are roughly parallel to each other, and roughly equal in length. This is a useful metric for line matching.
+ This algorithm is NOT equivalent to the standard Hausdorff distance.
+ However, it computes an approximation that is correct for a large subset of useful cases.
+ One important case is Linestrings that are roughly parallel to each other,
+ and roughly equal in length. This is a useful metric for line matching.
</para>
</note>
<para>Availability: 1.5.0</para>
@@ -918,26 +920,49 @@ The current implementation supports only vertices as the discrete locations. Thi
<refsection>
<title>Examples</title>
- <para>For each building, find the parcel that best represents it. First we require the parcel intersect with the geometry.
- DISTINCT ON guarantees we get each building listed only once, the ORDER BY .. ST_HausdorffDistance gives us a preference of parcel that is most similar to the building.</para>
- <programlisting>SELECT DISTINCT ON(buildings.gid) buildings.gid, parcels.parcel_id
- FROM buildings INNER JOIN parcels ON ST_Intersects(buildings.geom,parcels.geom)
- ORDER BY buildings.gid, ST_HausdorffDistance(buildings.geom, parcels.geom);</programlisting>
-
- <programlisting>postgis=# SELECT ST_HausdorffDistance(
- 'LINESTRING (0 0, 2 0)'::geometry,
- 'MULTIPOINT (0 1, 1 0, 2 1)'::geometry);
- st_hausdorffdistance
- ----------------------
- 1
-(1 row)
- </programlisting>
- <programlisting>postgis=# SELECT st_hausdorffdistance('LINESTRING (130 0, 0 0, 0 150)'::geometry, 'LINESTRING (10 10, 10 150, 130 10)'::geometry, 0.5);
- st_hausdorffdistance
+
+ <informalfigure>
+ <mediaobject>
+ <imageobject>
+ <imagedata fileref="images/st_hausdorffdistance01.png" />
+ </imageobject>
+ <caption><para>Hausdorff distance (red) and closest distance yellow)
+ between two lines </para></caption>
+ </mediaobject>
+ </informalfigure>
+
+<programlisting>
+SELECT ST_HausdorffDistance(geomA, geomB), ST_Distance(geomA, geomB)
+ FROM (SELECT 'LINESTRING (20 70, 70 60, 110 70, 170 70)'::geometry AS geomA,
+ 'LINESTRING (20 90, 130 90, 60 100, 190 100)'::geometry AS geomB) AS t;
+ st_hausdorffdistance | st_distance
+----------------------+-------------
+ 37.26206567625497 | 20
+</programlisting>
+
+<para><emphasis role="bold">Example:</emphasis> Hausdorff distance with densification.</para>
+<programlisting>
+SELECT ST_HausdorffDistance(
+ 'LINESTRING (130 0, 0 0, 0 150)'::geometry,
+ 'LINESTRING (10 10, 10 150, 130 10)'::geometry,
+ 0.5);
----------------------
- 70
-(1 row)
- </programlisting>
+ 70
+</programlisting>
+
+ <para><emphasis role="bold">Example:</emphasis>
+ For each building, find the parcel that best represents it.
+ First we require that the parcel intersect with the building geometry.
+ <code>DISTINCT ON</code> guarantees we get each building listed only once.
+ <code>ORDER BY .. ST_HausdorffDistance</code> selects the parcel that is most similar to the building.</para>
+
+<programlisting>
+SELECT DISTINCT ON (buildings.gid) buildings.gid, parcels.parcel_id
+ FROM buildings
+ INNER JOIN parcels
+ ON ST_Intersects(buildings.geom, parcels.geom)
+ ORDER BY buildings.gid, ST_HausdorffDistance(buildings.geom, parcels.geom);
+</programlisting>
</refsection>
<refsection>
-----------------------------------------------------------------------
Summary of changes:
doc/html/image_src/Makefile.in | 1 +
doc/html/image_src/st_hausdorffdistance01.wkt | 4 ++
doc/reference_measure.xml | 89 +++++++++++++++++----------
3 files changed, 62 insertions(+), 32 deletions(-)
create mode 100644 doc/html/image_src/st_hausdorffdistance01.wkt
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