Spatial Databases by Open Standards and Software 5

Spatial Databases by Open Standards and Software 5.Basics of PostGISCreated by XMLmind XSL-FO Converter. Gábor NagyCreated by XMLmind XSL-FO Converter. Spatial Databases by Open Standards and Software 5.: Basics of PostGISGábor NagyCreated by XMLmind XSL-FO Converter. Lector: Zoltán SikiCreated by XMLmind XSL-FO Converter. This module was created within TÁMOP - 4.1.2-08/1/A-2009-0027 "Tananyagfejlesztéssel a GEO-ért" ("Educational material development for GEO") project. The project was funded by the European Union and the Hungarian Government to the amount of HUF 44,706,488. v 1.0Publication date 2010Created by XMLmind XSL-FO Converter. Copyright ©Created by XMLmind XSL-FO Converter. 2010 University of West Hungary Faculty of GeoinformaticsAbstractPostGIS is a geospatial addition for the PostgreSQL, which is based in OGC 06-103r4 and OGC 04-104r4.Created by XMLmind XSL-FO Converter. The right to this intellectual property is protected by the 1999/LXXVI copyright law. Any unauthorized use of this material is prohibited. No part of this product may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system without express written permission from the author/publisher.Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. Table of ContentsCreated by XMLmind XSL-FO Converter. 5. Basics of PostGIS...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 1. 5.1 Introduction...... Created by XMLmind XSL-FO Converter. 02. 5.2 Installing PostGISCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 2.1. 5.2.1 Installing from compiled binariesCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 2.2. 5.2.2 Installing from source...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 2.3. 5.2.3 Installing to a database...... Created by XMLmind XSL-FO Converter. 03. 5.3 Creating spatial tablesCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 4. 5.4 Input and output functions...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 4.1. 5.4.1 GeomFromText...... Created by XMLmind XSL-FO Converter. 0Created by XMLmind XSL-FO Converter. 4.2. 5.4.2 GeomFromEWKT...... Created by XMLmind XSL-FO Converter. 04.3. 5.4.3 AsTextCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 4.4. 5.4.4 Binary input and output...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 4.5. 5.4.5 GML and KML input and output...... Created by XMLmind XSL-FO Converter. 05. 5.5 Calculating numerical attributes of geometryCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 5.1. 5.5.1 AreaCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 5.2. 5.5.2 Perimeter...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 5.3. 5.5.3 Length...... Created by XMLmind XSL-FO Converter. 06. 5.6 Calculating geometry valuesCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 6.1. 5.6.1 Buffer...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 6.2. 5.6.2 ConvexHull...... Created by XMLmind XSL-FO Converter. 06.3. 5.6.3 CentroidCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 6.4. 5.6.4 SimplifyCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 6.5. 5.6.5 Segmentize...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 7. 5.7 Relation of two geometries...... Created by XMLmind XSL-FO Converter. 07.1. 5.7.1 DistanceCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 7.2. 5.7.2 AzimuthCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 7.3. 5.7.3 Equals...... Created by XMLmind XSL-FO Converter. 07.4. 5.7.4 ContainsCreated by XMLmind XSL-FO Converter....... Created by XMLmind XSL-FO Converter. 07.5. 5.7.5 OverlapsCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 7.6. 5.7.6 Intersects...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 7.7. 5.7.7 Touches...... Created by XMLmind XSL-FO Converter. 07.8. 5.7.8 RelateCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 8. 5.8 Calculate derived geometry...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 8.1. 5.8.1 Transform...... Created by XMLmind XSL-FO Converter. 08.2. 5.8.2 AffineCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 8.3. 5.8.3 SnapToGridCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 9. 5.9 Set functions...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 9.1. 5.9.1 ST_Union...... Created by XMLmind XSL-FO Converter. 09.2. 5.9.2 IntersectionCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 9.3. 5.9.3 DifferenceCreated by XMLmind XSL-FO Converter...... 0Created by XMLmind XSL-FO Converter. 9.4. 5.9.4 SymDifference...... Created by XMLmind XSL-FO Converter. 010. 5.10 Aggregate functionsCreated by XMLmind XSL-FO Converter....... Created by XMLmind XSL-FO Converter. 010.1. 5.10.1 ST_UnionCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. 10.2. 5.10.2 Collect...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. 10.3. 5.10.3 Poligonize...... Created by XMLmind XSL-FO Converter. 011. 5.11 OperatorsCreated by XMLmind XSL-FO Converter....... 0Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. List of TablesCreated by XMLmind XSL-FO Converter. 1. Operators of PostGIS...... Created by XMLmind XSL-FO Converter.0Created by XMLmind XSL-FO Converter. Chapter 5. Basics of PostGISCreated by XMLmind XSL-FO Converter. 1. 5.1 IntroductionCreated by XMLmind XSL-FO Converter. The PostGIS is an additional package to PostgreSQL, which provides geographic objects and functions based on OGC 06-103r4 and OGC 06-104r4. The PostGIS (like PostgreSQL) is an open source software, developed by the Refractions Research.PostGIS uses some other open source softwares. PROJ.4 for the reprojection, and the GEOS (Geometry Engine, Open Source) for many geospatial functions.Created by XMLmind XSL-FO Converter. PostGIS provides geometry and geography type and functions for these types. Most of these functions are based on OGC 06-103r4 and OGC 06-104r4 standards, but PostGIS has some other functions.PostGIS 1.5.2 contains 10 types, 785 functions and 17 aggregate functions. The functions and aggregateCreated by XMLmind XSL-FO Converter. functions are started with “ST_” prefix, but most of them may be used without this prefix. The descriptions of the functions use the name with prefix, where we can not use the function without “ST_” (for example ST_UNION).Geography type is similar to geometry type. The objects of geography type are determined on a sphere orCreated by XMLmind XSL-FO Converter. spheroid. It is different from a geometry type with spherical reference system, because it uses spherical topology instead of plane.2. 5.2 Installing PostGISCreated by XMLmind XSL-FO Converter. 2.1. 5.2.1 Installing from compiled binariesCreated by XMLmind XSL-FO Converter. Under Linux operating system, the simplest solution is to use the package manager to install the PostGIS package of the distribution. For example use the apt-get install postgis command in a Debian based system. (The name of the required package may be different.)Created by XMLmind XSL-FO Converter. When we would like to install PostgreSQL to a Windows operating system, we can use more different installer programs. Some installers contain the PostGIS package, and it is selectable during the installation process.The PostgreSQL and PostGIS could be installed from a ZIP file too. Both of the downloaded files must be unpacked to a directory. The PostgreSQL programs are available from this directory.Created by XMLmind XSL-FO Converter. 2.2. 5.2.2 Installing from sourceThe PostgreSQL and the PostGIS are open source programs. The source code of these programs can be downloaded from the following home pages (http://www.postgresql.org and http://www.postgis.org).Created by XMLmind XSL-FO Converter. Both of these programs have regular build system, and could be installed by ./configure, Created by XMLmind XSL-FO Converter. make and make install commands. This work needs some other software. We can install this software by the package manager (need the development packages, with -dev ended name) or build from source also.Created by XMLmind XSL-FO Converter. 2.3. 5.2.3 Installing to a databaseThe installation of PostGIS to the database server is not sufficient, because PostGIS contains stored procedures. These stored procedures (and types) must be available in each database, where we should use the geospatial function of PostGIS.Created by XMLmind XSL-FO Converter. All of the necessary SQL commands, which create the required database objects, are in the postgis.sql file. This file is in the Created by XMLmind XSL-FO Converter. share/contrib/postgis-version directory, where version is the version of PostGIS.This file can be run from the psql console. For example:Created by XMLmind XSL-FO Converter. \i /usr/local/share/contrib/postgis-1.5/postgis.sqlWe can use the -f option of the psql command from the shell:Created by XMLmind XSL-FO Converter. psql -f /usr/local/share/contrib/postgis-1.5/postgis.sql gisdatabase gisuserAfter the execution of these commands, PostGIS is available in the database.Created by XMLmind XSL-FO Converter. The spatial_ref_sys table was created by the postgis.sqlCreated by XMLmind XSL-FO Converter. , but the table is empty. We can fill the descriptions of the spatial reference systems to this table by the spatial_ref_sys.sql file, which locates in the directory of the postgis.sql.Created by XMLmind XSL-FO Converter. 3. 5.3 Creating spatial tablesCreated by XMLmind XSL-FO Converter. PostGIS provides geometry and geography types. Geometry and geography columns can be created by the CREATE TABLESQL command, together with the other columns of the table:Created by XMLmind XSL-FO Converter. gisdb=# CREATE TABLE gistable (id serial PRIMARY KEY, name char(20),Created by XMLmind XSL-FO Converter. geom geometry);CREATE TABLECreated by XMLmind XSL-FO Converter. This is not a perfect solution, because many applications need one record in the geometry_columns table for each geometry type columns of the tables of the database. Use the AddGeomemertyColumnsCreated by XMLmind XSL-FO Converter. function instead to create the geometry type columns of a table, after the table has been created. This function can be called by a SELECT command. For example:Created by XMLmind XSL-FO Converter. gisdb=# CREATE TABLE gistable (id serial PRIMARY KEY, name char(20));Created by XMLmind XSL-FO Converter. CREATE TABLE gisdb=# SELECT AddGeometryColumn('gistable', 'geom', 23700, 'POLYGON', 2);Created by XMLmind XSL-FO Converter. addgeometrycolumn------ public.gistable.geom SRID:23700 TYPE:POLYGON DIMS:2Created by XMLmind XSL-FO Converter. (1 row)The first parameter of the function is the name of the table, the second parameter is the name of the new geometry column. The third parameter is the SRID (the identification number of the reference system), the fourth prameter is the type of the geometry, fifth parameter is the dimension of the geometry.Created by XMLmind XSL-FO Converter. AddGeomemertyColumn function inserts the necessary record into the geometry_columns table, and set constrains to check the geometry type, dimension and SRID (identification number of the spatial reference system).Created by XMLmind XSL-FO Converter. The spatial index is very important for effective spatial queries. GIST type indexes can be built for the geometry type columns: gisdb=# CREATE INDEX gistable_geom ON gistable USING GIST (geom);Created by XMLmind XSL-FO Converter. CREATE INDEXThe GIST (Generalized Search Tree) index provides an R-tree like data structure for fast queries on the basis of the bounding rectangles. The creation of this index is recommended for each geometry column, because it is very useful in many geospatial operations.Created by XMLmind XSL-FO Converter. The DropGeometryColumn function can be used to remove a geometry column from a table. For example:Created by XMLmind XSL-FO Converter. SELECT DropGeometryColumn('gistable','geom');4. 5.4 Input and output functionsCreated by XMLmind XSL-FO Converter. These functions provide the creation of geometry objects from text and binary data, and dump the geometry to various text and binary formats.Created by XMLmind XSL-FO Converter. 4.1. 5.4.1 GeomFromTextThe GeomFromTextCreated by XMLmind XSL-FO Converter. or GeometryFromText function returns a geometry value.GeomFromText('POLYGON((4 4,4 7,7 7,7 4,4 4))', 23700)Created by XMLmind XSL-FO Converter. The first parameter is the WKT representation of the geometry, the second parameter is the identification number of the spatial reference system (SRID).Created by XMLmind XSL-FO Converter. The ST_GeogFromText is similar to the GeomFromText, but creates geography values. This function needs only one parameter, the WKT representation of the geography; the SRID will be 4326 (WGS84).Created by XMLmind XSL-FO Converter. 4.2. 5.4.2 GeomFromEWKTCreated by XMLmind XSL-FO Converter. The GeomFromEWKT is similar to the GeomFromText function, but uses EWKT (Extended WKT) representation.Created by XMLmind XSL-FO Converter. GeomFromEWKT('SRID=23700;POLYGON((4 4,4 7,7 7,7 4,4 4))')This function has only one parameter: The EWKT representation text of the geometry, which contains the SRID too.Created by XMLmind XSL-FO Converter. 4.3. 5.4.3 AsTextCreated by XMLmind XSL-FO Converter. AsTex t function returns the WKT representation of the geometry. The AsEWKT is very similar, but the result contains the SRID of the geometry too. Both of these functions have one parameter: one geometry value.Created by XMLmind XSL-FO Converter. 4.4. 5.4.4 Binary input and outputPostGIS has functions for the binary input and output. These functions are very similar to the text functions, butCreated by XMLmind XSL-FO Converter. use WKB or EWKB format for the input (GeomFromWKB, GeomFromEWBK) or the output (Created by XMLmind XSL-FO Converter. AsBinary, AsEWKB).Created by XMLmind XSL-FO Converter. 4.5. 5.4.5 GML and KML input and outputPostGIS can handle GML and KML data. The AsGMLCreated by XMLmind XSL-FO Converter. and the AsKML functions return the representation of the geometry in these XML based formats. The result is a text value. This text value contains the XML description of the geometry in the required format:Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsGML(GeomFromText('POLYGON((4 4,4 7,7 7,7 4,4 4))', 4326)); asgmlCreated by XMLmind XSL-FO Converter. ------<gml:Polygon srsName="EPSG:4326"><gml:outerBoundaryIs><gml:LinearRing>Created by XMLmind XSL-FO Converter. <gml:coordinates>4,4 4,7 7,7 7,4 4,4</gml:coordinates></gml:LinearRing>Created by XMLmind XSL-FO Converter. </gml:outerBoundaryIs></gml:Polygon>(1 row) gisdb=# SELECT AsKML(GeomFromText('POLYGON((4 4,4 7,7 7,7 4,4 4))', 4326));Created by XMLmind XSL-FO Converter. askml------Created by XMLmind XSL-FO Converter. <Polygon><outerBoundaryIs><LinearRing><coordinates>4,4 4,7 7,7 7,4 4,4</coordinates></LinearRing></outerBoundaryIs></Polygon>Created by XMLmind XSL-FO Converter. (1 row)The Created by XMLmind XSL-FO Converter. GeomFromGML and GeomFromKML functions provide the data input from these formats.Created by XMLmind XSL-FO Converter. 5. 5.5 Calculating numerical attributes of geometryCreated by XMLmind XSL-FO Converter. 5.1. 5.5.1 AreaArea function returns the area of a Polygon or MultiPolygon geometry or the total area of the polygon elements of a GeometryCollection. If the parameter of the function has a different geometry type the result will be zero.Created by XMLmind XSL-FO Converter. gisdb=# SELECT ST_Area('POLYGON((1 1,1 4,4 4,4 1,1 1))'); st_areaCreated by XMLmind XSL-FO Converter. ------9Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT ST_Area('POLYGON((1 1,1 4,4 4,4 1,1 1),Created by XMLmind XSL-FO Converter. (2 2,2 3,3 3,3 2,2 2))'); st_area------Created by XMLmind XSL-FO Converter. 8(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT ST_Area('LINESTRING(4 5,7 9,11 2)'); st_areaCreated by XMLmind XSL-FO Converter. ------0Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT ST_Area('GEOMETRYCOLLECTION(Created by XMLmind XSL-FO Converter. POLYGON((1 1,1 4,4 4,4 1,1 1)),POINT(7 8))'); st_area------Created by XMLmind XSL-FO Converter. 9(1 row)Created by XMLmind XSL-FO Converter. The parameter of the ST_Area function (as in the examples above) may be a text value with the WKT representation of the geometry too. This solution will be applied in some of the following examples of other functions.Created by XMLmind XSL-FO Converter. 5.2. 5.5.2 PerimeterPerimeterCreated by XMLmind XSL-FO Converter. function returns the perimeter (length of the boundary) of a Polygon or MultiPolygon geometry or the the total perimeter of the polygon elements of a GeometryCollection. If the parameter of the function has a different geometry type the result will be zero. gisdb=# SELECT Perimeter('POLYGON((1 1,1 4,4 4,4 1,1 1))');Created by XMLmind XSL-FO Converter. perimeter------12Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Perimeter('POLYGON((1 1,1 4,4 4,4 1,1 1),Created by XMLmind XSL-FO Converter. (2 2,2 3,3 3,3 2,2 2))'); perimeterCreated by XMLmind XSL-FO Converter. ------16Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Perimeter('LINESTRING(4 5,7 9,11 2)');Created by XMLmind XSL-FO Converter. perimeter------0Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Perimeter('GEOMETRYCOLLECTION(Created by XMLmind XSL-FO Converter. POLYGON((1 1,1 4,4 4,4 1,1 1)),POINT(7 8))'); perimeterCreated by XMLmind XSL-FO Converter. ------12(1 row)Created by XMLmind XSL-FO Converter. The Perimeter3D function works similarly to the Perimeter function, but calculates the 3D perimeter, if the geometry has Z coordinates. The Created by XMLmind XSL-FO Converter. Perimeter2D function always returns the 2D perimeter. gisdb=# SELECT Perimeter2D('POLYGON((1 1 0,1 4 0,4 4 4,Created by XMLmind XSL-FO Converter. 4 1 0,1 1 0))'); perimeter2dCreated by XMLmind XSL-FO Converter. ------12(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Perimeter3D('POLYGON((1 1 0,1 4 0,4 4 4,4 1 0,1 1 0))');Created by XMLmind XSL-FO Converter. perimeter3d------Created by XMLmind XSL-FO Converter. 16(1 row)Created by XMLmind XSL-FO Converter. 5.3. 5.5.3 LengthLength function returns the length of a LineString or MultiLineString geometry or the the total length of the linestringCreated by XMLmind XSL-FO Converter. elements of a GeometryCollection. If the parameter of the function has a different geometry type the result will be zero. gisdb=# SELECT ST_Length('LINESTRING(1 2,5 2,9 5)');Created by XMLmind XSL-FO Converter. st_length------9Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT ST_Length('MULTILINESTRING((1 2,5 2,9 5),Created by XMLmind XSL-FO Converter. (1 4,3 4))'); st_lengthCreated by XMLmind XSL-FO Converter. ------11(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT ST_Length('POLYGON((1 1,1 4,4 4,4 1,1 1))'); st_lengthCreated by XMLmind XSL-FO Converter. ------0Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT ST_Length('GEOMETRYCOLLECTION(Created by XMLmind XSL-FO Converter. LINESTRING(1 4,1 10),POINT(7 8))'); st_lengthCreated by XMLmind XSL-FO Converter. ------6(1 row)Created by XMLmind XSL-FO Converter. The “length” is also the name of the function, which returns the length of a text. PostgreSQL makes a distinction between the two length functions by the type of the parameter. We have to use GeomFromText function in the LengthCreated by XMLmind XSL-FO Converter. function, or use ST_Length function.Created by XMLmind XSL-FO Converter. Length3D function works similarly to the Length function, but calculates the 3D perimeter, if the geometry has Z coordinates. gisdb=# SELECT ST_Length('LINESTRING(1 3 2,5 15 5)');Created by XMLmind XSL-FO Converter. st_length------Created by XMLmind XSL-FO Converter. 12.6491106406735(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT ST_Length2D('LINESTRING(1 3 2,5 15 5)'); st_length2d------Created by XMLmind XSL-FO Converter. 12.6491106406735(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT ST_Length3D('LINESTRING(1 3 2,5 15 5)'); st_length3dCreated by XMLmind XSL-FO Converter. ------13Created by XMLmind XSL-FO Converter. (1 row)Length_Spheroid(and Lebgth3D_SpheroidCreated by XMLmind XSL-FO Converter. ) function can calculate the length of the geometry on a spheroid (sphere or ellipsoid). gisdb=# SELECT ST_Length_Spheroid('LINESTRING(18.41281 47.18458,Created by XMLmind XSL-FO Converter. 18.41883 47.18878,18.42706 47.19097)','SPHEROID["WGS84",6378137,298.257223563]'); st_length_spheroidCreated by XMLmind XSL-FO Converter. ------1322.39548875418Created by XMLmind XSL-FO Converter. (1 row)6. 5.6 Calculating geometry valuesCreated by XMLmind XSL-FO Converter. These functions return a new geometry, which are built from the original geometries and sometimes other parameters.Created by XMLmind XSL-FO Converter. 6.1. 5.6.1 BufferBuffer function returns a Polygon or MultiPolygon geometry that represents the set of the points whose distance from a geometry specified by the first parameter are less than or equal to the distance specified by the second parameter.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Buffer('POINT(3 4)',1)); astextCreated by XMLmind XSL-FO Converter. ------POLYGON((4 4,3.981 3.805,3.924 3.617,3.831 3.444,3.707 3.293,Created by XMLmind XSL-FO Converter. 3.556 3.169,3.383 3.076,3.195 3.019,3 3,2.805 3.019,2.617 3.076,2.444 3.169,2.293 3.293,2.169 3.444,2.076 3.617,2.019 3.805,2 4,Created by XMLmind XSL-FO Converter. 2.019 4.195,2.076 4.383,2.169 4.556,2.293 4.707,2.444 4.831,2.617 4.924,2.805 4.981,3 5,Created by XMLmind XSL-FO Converter. 3.195 4.981,3.383 4.924,3.556 4.831,3.707 4.707,3.831 4.556,3.924 4.383,3.981 4.195,4 4))(1 row)Created by XMLmind XSL-FO Converter. By definition, the boundary of the shape of the buffer contains arc sections, but the Polygon and MultiPolygon geometries are limited to have linear rings. The arc sections will be replaced by more linear sections. By default, a quarter of a circle will be replaced by 8 line segments. The optionally third parameter of the function specifies the number of line segments or other parameters of the buffer generation.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ST_Buffer('POINT(3 4)',1,'quad_segs=2')); astextCreated by XMLmind XSL-FO Converter. ------POLYGON((4 4,3.707 3.293,3 3,2.293 3.293,2 4,2.293 4.707,3 5,3.707 4.707,4 4))Created by XMLmind XSL-FO Converter. (1 row)6.2. 5.6.2 ConvexHullCreated by XMLmind XSL-FO Converter. ConvexHull function returns a geometry that represents the least convex point set, which contains all the points of a geometry, which were specified by the parameter of the function.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ConvexHull('POINT(3 5)')); astextCreated by XMLmind XSL-FO Converter. ------POINT(3 5)(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ConvexHull('LINESTRING(2 3,5 8)')); astextCreated by XMLmind XSL-FO Converter. ------LINESTRING(2 3,5 8)Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(ConvexHull('MULTIPOINT(2 2,4 5)'));Created by XMLmind XSL-FO Converter. astext------LINESTRING(2 2,4 5)Created by XMLmind XSL-FO Converter. (1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ConvexHull('MULTIPOINT(2 2,4 5,3 4)')); astext------Created by XMLmind XSL-FO Converter. POLYGON((2 2,3 4,4 5,2 2))(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ConvexHull('POLYGON((1 1,1 4,4 4,4 1,1 1),(2 2,2 3,3 3,3 2,2 2))'));Created by XMLmind XSL-FO Converter. astext------POLYGON((1 1,1 4,4 4,4 1,1 1))Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(ConvexHull('POLYGON((1 5,6 9,4 6,8 2,1 5))'));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. POLYGON((8 2,1 5,6 9,8 2))(1 row)Created by XMLmind XSL-FO Converter. 6.3. 5.6.3 CentroidCentroidCreated by XMLmind XSL-FO Converter. function returns the geometric centre of the geometry as a Point geometry. gisdb=# SELECT AsText(Centroid('POLYGON((1 1,1 4,4 4,4 1,1 1))')); astextCreated by XMLmind XSL-FO Converter. ------POINT(2.5 2.5)Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(Centroid('POLYGON((1 1,1 4,4 4,4 1,1 1),Created by XMLmind XSL-FO Converter. (2 2,2 3,3 3,3 2,2 2))')); astextCreated by XMLmind XSL-FO Converter. ------POINT(2.5 2.5)(1 row)Created by XMLmind XSL-FO Converter. 6.4. 5.6.4 SimplifyCreated by XMLmind XSL-FO Converter. Simplify function returns a simplified variant of the geometry, which is the first parameter of the function. This function uses the Douglas-Peuker algorithm, with a tolerance, which is the second parameter. gisdb=# SELECT AsText(Simplify('LINESTRING(2 4,4 7,7 9)', 1));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. LINESTRING(2 4,7 9)(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Simplify('LINESTRING(2 4,4 7,7 9)', 0.5)); astext------Created by XMLmind XSL-FO Converter. LINESTRING(2 4,4 7,7 9)(1 row)Created by XMLmind XSL-FO Converter. ST_SiplifyPreserveTopology function is the same as the Simplify, but preserves the topology relations of parts of the object (linear rings, linestring segments).Created by XMLmind XSL-FO Converter. SELECT AsText(Simplify(ST_GeomFromText('MULTILINESTRING((1 5,5 7,9 5),(1 1,5 6,9 1))'),2));Created by XMLmind XSL-FO Converter. astext------MULTILINESTRING((1 5,9 5),(1 1,5 6,9 1))Created by XMLmind XSL-FO Converter. (1 row)SELECT AsText(ST_SimplifyPreserveTopology(ST_GeomFromText(Created by XMLmind XSL-FO Converter. 'MULTILINESTRING((1 5,5 7,9 5),(1 1,5 6,9 1))'),2)); astext------Created by XMLmind XSL-FO Converter. MULTILINESTRING((1 5,5 7,9 5),(1 1,5 6,9 1))(1 row)Created by XMLmind XSL-FO Converter. 6.5. 5.6.5 SegmentizeSegmentizeCreated by XMLmind XSL-FO Converter. function returns a modified variant of the geometry (the first parameter), which has no longer segments than a given distance (the second parameter). gisdb=# SELECT AsText(Segmentize('LINESTRING(1 3,1 6)',3));Created by XMLmind XSL-FO Converter. astext------LINESTRING(1 3,1 6)Created by XMLmind XSL-FO Converter. (1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Segmentize('LINESTRING(1 3,1 9)',3)); astext------Created by XMLmind XSL-FO Converter. LINESTRING(1 3,1 6,1 9)(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Segmentize('LINESTRING(1 3,1 9)',2)); astextCreated by XMLmind XSL-FO Converter. ------LINESTRING(1 3,1 5,1 7,1 9)(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Segmentize('POLYGON((3 2,3 8,5 8,5 2,3 2))',3));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. POLYGON((3 2,3 5,3 8,5 8,5 5,5 2,3 2))(1 row)Created by XMLmind XSL-FO Converter. 7. 5.7 Relation of two geometriesCreated by XMLmind XSL-FO Converter. 7.1. 5.7.1 DistanceDistance function returns the shortest distance between two geometries, which are the parameters of the function. The result is zero, if the geometries have a common point.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Distance('POINT(1 2)','POINT(4 6)'); distanceCreated by XMLmind XSL-FO Converter. ------5Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Distance('LINESTRING(2 2,8 2)','POINT(4 6)');Created by XMLmind XSL-FO Converter. distance------4Created by XMLmind XSL-FO Converter. (1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Distance('POLYGON((2 2,2 3,8 3,8 2,2 2))','POINT(4 6)'); distanceCreated by XMLmind XSL-FO Converter. ------3Created by XMLmind XSL-FO Converter. (1 row)Distance function can be used between geography objects too.Created by XMLmind XSL-FO Converter. gisdb=# SELECT ST_Distance(ST_GeogFromText('SRID=4326;POINT(18.41883 47.18878)'),Created by XMLmind XSL-FO Converter. ST_GeogFromText('SRID=4326;POINT(18.41281 47.18458)')); st_distance------Created by XMLmind XSL-FO Converter. 652.839475007109(1 row)Created by XMLmind XSL-FO Converter. ST_ShortestLine function returns the shortest line segment between two geometries as a LineString geometry. The length of this LineString equals to the result of the Distance function.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ST_ShortestLine('POINT(1 2)','POINT(4 6)')); astextCreated by XMLmind XSL-FO Converter. ------LINESTRING(1 2,4 6)Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(ST_ShortestLine('LINESTRING(2 2,8 2)',Created by XMLmind XSL-FO Converter. 'POINT(4 6)')); astext------Created by XMLmind XSL-FO Converter. LINESTRING(4 2,4 6)(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ST_ShortestLine('POLYGON((2 2,2 3,8 3,8 2,2 2))','POINT(4 6)'));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. LINESTRING(4 3,4 6)(1 row)Created by XMLmind XSL-FO Converter. 7.2. 5.7.2 AzimuthAzimuth function returns the azimuth in radians from the point specified in the first parameter to the point specified in the second parameter. If any parameter has no Point geometry, the function returns NULL.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Azimuth('POINT(1 1)','POINT(1 2)'); azimuthCreated by XMLmind XSL-FO Converter. ------0Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Azimuth('POINT(1 1)','POINT(2 2)');Created by XMLmind XSL-FO Converter. azimuth------Created by XMLmind XSL-FO Converter. 0.785398163397448(1 row)Created by XMLmind XSL-FO Converter. 7.3. 5.7.3 EqualsEquals function returns true value, if the point set of the geometries (specified by the parameters of the function) are spatially equal.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Equals('POINT(2 4)','POINT(3 5)'); equalsCreated by XMLmind XSL-FO Converter. ------ fCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Equals('POINT(2 4)','POINT(2 4)');Created by XMLmind XSL-FO Converter. equals------ tCreated by XMLmind XSL-FO Converter. (1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Equals('MULTIPOINT(2 4,5 8)','MULTIPOINT(5 8,2 4)'); equals------Created by XMLmind XSL-FO Converter. t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Equals('LINESTRING(2 4,5 8)','LINESTRING(5 8,2 4)'); equalsCreated by XMLmind XSL-FO Converter. ------ t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Equals('POLYGON((1 1,1 5,5 1,1 1))','POLYGON((1 5,1 1,5 1,1 5))');Created by XMLmind XSL-FO Converter. equals------Created by XMLmind XSL-FO Converter. t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Equals('POLYGON((2 2,2 4,4 4,4 2,2 2))','POLYGON((2 2,2 3,2 4,4 4,4 2,2 2))');Created by XMLmind XSL-FO Converter. equals------ tCreated by XMLmind XSL-FO Converter. (1 row)7.4. 5.7.4 ContainsCreated by XMLmind XSL-FO Converter. Contains function returns true value, if the geometry specified by the first parameter contains all the points of the geometry specified by the second parameter.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Contains('POLYGON((1 1,1 5,5 5,5 1,1 1))','POINT(3 3)');Created by XMLmind XSL-FO Converter. contains------ tCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Contains('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POINT(8 4)'); containsCreated by XMLmind XSL-FO Converter. ------ f(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Contains('POLYGON((1 1,1 5,5 5,5 1,1 1))','LINESTRING(2 2,3 4)');Created by XMLmind XSL-FO Converter. contains------Created by XMLmind XSL-FO Converter. t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Contains('POLYGON((1 1,1 5,5 5,5 1,1 1))','LINESTRING(2 4,6 4)');Created by XMLmind XSL-FO Converter. contains------ fCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Contains('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POLYGON((2 2,2 3,3 3,3 2,2 2))'); containsCreated by XMLmind XSL-FO Converter. ------ t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Contains('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POLYGON((4 4,4 7,7 7,7 4,4 4))'); contains------Created by XMLmind XSL-FO Converter. f(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Contains('LINESTRING(2 3,4 3)','POINT(3 3)');Created by XMLmind XSL-FO Converter. contains------ tCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Contains('LINESTRING(2 3,7 3)',Created by XMLmind XSL-FO Converter. 'LINESTRING(3 3,4 3)'); containsCreated by XMLmind XSL-FO Converter. ------ tCreated by XMLmind XSL-FO Converter. (1 row)7.5. 5.7.5 OverlapsCreated by XMLmind XSL-FO Converter. Overlaps function returns true value, if the common set of the points of the two geometries has the same dimension as the objects. This function returns true value, if the two geometries have common points, but both of the geometries have points, which are not contained by the other object. This function does not work with GeometryCollection objects.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Overlaps('POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((2 2,2 3,3 3,3 2,2 2))'); overlapsCreated by XMLmind XSL-FO Converter. ------ fCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Overlaps('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POLYGON((4 4,4 7,7 7,7 4,4 4))'); overlapsCreated by XMLmind XSL-FO Converter. ------ t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Overlaps('LINESTRING(2 3,7 3)','LINESTRING(3 3,4 3)');Created by XMLmind XSL-FO Converter. overlaps------Created by XMLmind XSL-FO Converter. f(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Overlaps('LINESTRING(2 3,7 3)','LINESTRING(1 3,4 3)'); overlapsCreated by XMLmind XSL-FO Converter. ------ tCreated by XMLmind XSL-FO Converter. (1 row)7.6. 5.7.6 IntersectsCreated by XMLmind XSL-FO Converter. Intersects function returns true value, if the two geometries have at least one common point.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Intersects('POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((4 4,4 7,7 7,7 4,4 4))'); intersectsCreated by XMLmind XSL-FO Converter. ------ tCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Intersects('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'LINESTRING(2 4,6 4)'); intersects------Created by XMLmind XSL-FO Converter. t(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT Intersects('POLYGON((1 1,1 5,5 5,5 1,1 1))','LINESTRING(6 4,7 4)'); intersectsCreated by XMLmind XSL-FO Converter. ------ fCreated by XMLmind XSL-FO Converter. (1 row)7.7. 5.7.7 TouchesCreated by XMLmind XSL-FO Converter. Touches function returns true value, if the two geometries have common points, but these points are located only on the boundaries of the objects.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Touches('POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((5 2,5 4,7 4,7 2,5 2))'); touchesCreated by XMLmind XSL-FO Converter. ------ tCreated by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT Touches('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POLYGON((5 2,4 4,7 4,7 2,5 2))'); touchesCreated by XMLmind XSL-FO Converter. ------ f(1 row)Created by XMLmind XSL-FO Converter. 7.8. 5.7.8 RelateCreated by XMLmind XSL-FO Converter. Relate function returns a text, which contains the element of the DE-9IM. gisdb=# SELECT Relate('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POLYGON((5 2,5 4,7 4,7 2,5 2))'); relate------Created by XMLmind XSL-FO Converter. FF2F11212(1 row)Created by XMLmind XSL-FO Converter. If the Relate function has three parameters, the third parameter is a DE-9IM pattern. The function returns true, if the pattern matches the relation of the two geometries, given in the first and second parameters.Created by XMLmind XSL-FO Converter. gisdb=# SELECT Relate('POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((5 2,5 4,7 4,7 2,5 2))','****1****');Created by XMLmind XSL-FO Converter. relate------ tCreated by XMLmind XSL-FO Converter. (1 row)8. 5.8 Calculate derived geometryCreated by XMLmind XSL-FO Converter. These functions return a modified geometry preserving the topology of the original object.Created by XMLmind XSL-FO Converter. 8.1. 5.8.1 TransformPostGIS can transform geometries between spatial reference systems. The geometries contain the identification number of spatial reference system, further on SRID. The spatial_ref_sys table contains the descriptions of spatial reference systems identified by the SRID.Created by XMLmind XSL-FO Converter. Transform function returns a transformed geometry from the original reference system (specified as the first parameter, SRID is involved in the geometry) to a new reference system, which SRID is specified by the second parameter.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Transform('SRID=4326;POINT(18.4183 47.1888)',23700));Created by XMLmind XSL-FO Converter. astext------POINT(602233.50006042 205129.440451882)Created by XMLmind XSL-FO Converter. (1 row)4326 is the SRID of WGS84, 23700 is the SRID of Hungarian Projection System (EOV).Created by XMLmind XSL-FO Converter. The SRID of an object can be modified without transformation by the SetSRID function.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ST_SetSRID('SRID=4326;POINT(18.4183 47.1888)',23700));Created by XMLmind XSL-FO Converter. astext------POINT(18.4183 47.1888)Created by XMLmind XSL-FO Converter. (1 row)8.2. 5.8.2 AffineCreated by XMLmind XSL-FO Converter. Affine function returns an affine transformed geometry from the input geometry specified as the first parameter. The other 6 or 12 parameters of the function contain the parameters of a 2D or 3D transformation.Created by XMLmind XSL-FO Converter. 8.3. 5.8.3 SnapToGridSnapToGridCreated by XMLmind XSL-FO Converter. function returns modified geometry snapping points to a grid. The resolution of the grid is specified by the second parameter. The coordinates of a geometry can be rounded up by this function. gisdb=# SELECT AsText(SnapToGrid(Created by XMLmind XSL-FO Converter. 'POINT(602233.50006042 205129.440451882)',0.01)); astext------Created by XMLmind XSL-FO Converter. POINT(602233.5 205129.44)(1 row)Created by XMLmind XSL-FO Converter. 9. 5.9 Set functionsThese functions return a geometry which represents a point set which is the result of a set operation between two geometries.Created by XMLmind XSL-FO Converter. 9.1. 5.9.1 ST_UnionCreated by XMLmind XSL-FO Converter. ST_Union function returns a geometry that represents the union of the two geometries, which are specified as the parameters of the function.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(ST_Union('POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((3 3,3 7,7 7,7 3,3 3))'));Created by XMLmind XSL-FO Converter. astext------POLYGON((1 1,1 5,3 5,3 7,7 7,7 3,5 3,5 1,1 1))Created by XMLmind XSL-FO Converter. (1 row)Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. Figure 1. Union of two POLYGON geometries gisdb=# SELECT AsText(ST_Union('LINESTRING(2 3,5 3)','LINESTRING(4 3,7 3)')); astextCreated by XMLmind XSL-FO Converter. ------MULTILINESTRING((2 3,4 3),(4 3,5 3),(5 3,7 3))Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(ST_Union('POINT(2 4)','POINT(7 3)'));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. MULTIPOINT(2 4,7 3)(1 row)Created by XMLmind XSL-FO Converter. 9.2. 5.9.2 IntersectionIntersection function returns a geometry that represents the common parts of the two geometries, which are specified as the parameters of the function.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Intersection('POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((3 3,3 7,7 7,73,3 3))'));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. POLYGON((3 5,5 5,5 3,3 3,3 5))(1 row)Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. Figure 2. Intersection of two POLYGON geometries gisdb=# SELECT AsText(Intersection('LINESTRING(2 3,5 3)',Created by XMLmind XSL-FO Converter. 'LINESTRING(4 3,7 3)')); astext------Created by XMLmind XSL-FO Converter. LINESTRING(4 3,5 3)(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Intersection('POLYGON((1 1,1 5,5 5,5 1,1 1))','LINESTRING(3 4,8 9)')); astextCreated by XMLmind XSL-FO Converter. ------LINESTRING(3 4,4 5)Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(Intersection('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'MULTIPOINT(2 4,6 2)')); astextCreated by XMLmind XSL-FO Converter. ------POINT(2 4)(1 row)Created by XMLmind XSL-FO Converter. 9.3. 5.9.3 DifferenceCreated by XMLmind XSL-FO Converter. Difference function returns a geometry that represents that part of the first geometry, which is not in the second geometry. gisdb=# SELECT AsText(Difference('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'POLYGON((3 3,3 7,7 7,7 3,3 3))')); astext------Created by XMLmind XSL-FO Converter. POLYGON((1 1,1 5,3 5,3 3,5 3,5 1,1 1))(1 row)Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. Figure 3. Difference of two POLYGON geometries gisdb=# SELECT AsText(Difference('LINESTRING(2 3,5 3)','LINESTRING(4 3,7 3)')); astextCreated by XMLmind XSL-FO Converter. ------LINESTRING(2 3,4 3)Created by XMLmind XSL-FO Converter. (1 row) gisdb=# SELECT AsText(Difference('POLYGON((1 1,1 5,5 5,5 1,1 1))',Created by XMLmind XSL-FO Converter. 'LINESTRING(3 4,8 9)')); astext------Created by XMLmind XSL-FO Converter. POLYGON((1 1,1 5,4 5,5 5,5 1,1 1))(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Difference('POLYGON((1 1,1 5,5 5,5 1,1 1))','MULTIPOINT(2 4,6 2)')); astext------Created by XMLmind XSL-FO Converter. POLYGON((1 1,1 5,5 5,5 1,1 1))(1 row)Created by XMLmind XSL-FO Converter. 9.4. 5.9.4 SymDifferenceSymDifferenceCreated by XMLmind XSL-FO Converter. function returns a geometry that represents that part of the two input geometry, which belongs to only one of them. gisdb=# SELECT AsText(SymDifference(Created by XMLmind XSL-FO Converter. 'POLYGON((1 1,1 5,5 5,5 1,1 1))','POLYGON((3 3,3 7,7 7,7 3,3 3))')); astextCreated by XMLmind XSL-FO Converter. ------MULTIPOLYGON(((1 1,1 5,3 5,3 3,5 3,5 1,1 1)),Created by XMLmind XSL-FO Converter. ((5 3,5 5,3 5,3 7,7 7,7 3,5 3)))(1 row)Created by XMLmind XSL-FO Converter. Created by XMLmind XSL-FO Converter. Figure 4. Symmetric difference of two POLYGON geometries gisdb=# SELECT AsText(SymDifference('LINESTRING(2 3,5 3)',Created by XMLmind XSL-FO Converter. 'LINESTRING(4 3,7 3)')); astextCreated by XMLmind XSL-FO Converter. ------MULTILINESTRING((2 3,4 3),(5 3,7 3))(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(SymDifference('POLYGON((1 1,1 5,5 5,5 1,1 1))','LINESTRING(3 4,8 9)'));Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. GEOMETRYCOLLECTION(LINESTRING(4 5,8 9),POLYGON((1 1,1 5,4 5,5 5,5 1,1 1)))(1 row)Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(SymDifference(Created by XMLmind XSL-FO Converter. 'POLYGON((1 1,1 5,5 5,5 1,1 1))','MULTIPOINT(2 4,6 2)')); astext------Created by XMLmind XSL-FO Converter. GEOMETRYCOLLECTION(POINT(6 2),POLYGON((1 1,1 5,5 5,5 1,1 1)))Created by XMLmind XSL-FO Converter. (1 row)10. 5.10 Aggregate functionsCreated by XMLmind XSL-FO Converter. PostGIS’ aggregate functions create geometry from more geometries. These are used, where the usual SQL aggregate functions, for example in queries containing GROUP BY statement.10.1. 5.10.1 ST_UnionCreated by XMLmind XSL-FO Converter. ST_Union aggregate returns a geometry that represents the union of all input geometries.Created by XMLmind XSL-FO Converter. gisdb=# SELECT id, AsText(geom) FROM polygons; id | astext----+------Created by XMLmind XSL-FO Converter. 1 | POLYGON((1 1,1 5,5 5,5 1,1 1))2 | POLYGON((3 3,3 7,7 7,7 3,3 3))3 | POLYGON((8 8,8 9,9 9,9 8,8 8))Created by XMLmind XSL-FO Converter. (3 rows) gisdb=# SELECT AsText(ST_Union(geom)) FROM polygons;Created by XMLmind XSL-FO Converter. astext------Created by XMLmind XSL-FO Converter. MULTIPOLYGON(((8 8,8 9,9 9,9 8,8 8)),((1 1,1 5,3 5,3 7,7 7,7 3,5 3,5 1,1 1)))Created by XMLmind XSL-FO Converter. (1 row)10.2. 5.10.2 CollectCreated by XMLmind XSL-FO Converter. Collect aggregate returns a Multi* geometry or a GeometryCollection, which contains the elements of the original geometries. In contrast to the ST_Union, this aggregate does not do any spatial operation between the geometries.Created by XMLmind XSL-FO Converter. gisdb=# SELECT AsText(Collect(geom)) FROM polygons; astextCreated by XMLmind XSL-FO Converter. ------MULTIPOLYGON(((1 1,1 5,5 5,5 1,1 1)),((3 3,3 7,7 7,7 3,3 3)),Created by XMLmind XSL-FO Converter. ((8 8,8 9,9 9,9 8,8 8)))(1 row)Created by XMLmind XSL-FO Converter. 10.3. 5.10.3 PoligonizePoligonize aggregate returns a GeometryCollection that contains the possible Polygons, which can be created from the input LineString and MultiLineString objects.Created by XMLmind XSL-FO Converter. gisdb=# SELECT id, AsText(geom) from linework; id | astextCreated by XMLmind XSL-FO Converter. ----+------1 | LINESTRING(1 1,1 2)Created by XMLmind XSL-FO Converter. 2 | LINESTRING(1 2,2 2)3 | LINESTRING(2 2,2 1)4 | LINESTRING(2 1,1 1)Created by XMLmind XSL-FO Converter. 5 | LINESTRING(2 2,3 2)6 | LINESTRING(3 2,3 3)Created by XMLmind XSL-FO Converter. 7 | LINESTRING(3 3,2 3)8 | LINESTRING(2 3,2 2)Created by XMLmind XSL-FO Converter. (8 rows) gisdb=# SELECT AsText(Polygonize(geom)) from linework;Created by XMLmind XSL-FO Converter. astext------GEOMETRYCOLLECTION(Created by XMLmind XSL-FO Converter. POLYGON((1 1,1 2,2 2,2 1,1 1)),POLYGON((3 2,2 2,2 3,3 3,3 2)))Created by XMLmind XSL-FO Converter. (1 row)11. 5.11 OperatorsCreated by XMLmind XSL-FO Converter. PostGIS has some operators that return true or false values depending on the spatial relationship of the bounding boxes of the operands. These operators use spatial indexes of any operands.Created by XMLmind XSL-FO Converter. Table 1. Operators of PostGIS returns TRUE if A's bounding box overlaps B's A && BCreated by XMLmind XSL-FO Converter. A &A &<| returns TRUE if A's bounding box overlaps or is below B's BCreated by XMLmind XSL-FO Converter. A &> B returns TRUE if A' bounding box overlaps or is to the right of B'sA <Created by XMLmind XSL-FO Converter. A <<| returns TRUE if A's bounding box is strictly below B's BA = B returns TRUE if A's bounding box is the same as B'sCreated by XMLmind XSL-FO Converter. A >> B returns TRUE if A's bounding box is strictly to the right of B'sA @ B returns TRUE if A's bounding box is contained by B'sA |&> Created by XMLmind XSL-FO Converter. B returns TRUE if A's bounding box overlaps or is above B'sA |>> returns TRUE if A's bounding box is strictly above B's BA ~ B returns TRUE if A's bounding box contains B'sCreated by XMLmind XSL-FO Converter. A ~= B returns TRUE if A's bounding box is the same as B'sThese operators are very useful for fast pre-screening in spatial queries.Created by XMLmind XSL-FO Converter. BibliographyCreated by XMLmind XSL-FO Converter. PostgreSQL Global Development Group:Created by XMLmind XSL-FO Converter. 1996-2010.Refractions Research Inc.:Created by XMLmind XSL-FO Converter. PostGIS 1.5.2 manual,2010.Created by XMLmind XSL-FO Converter. Open Geospatial Consortium:OpenGIS Implementation Standard for Geographic information - Simple feature access - Part 1: Common architecture, OGC 06-103r4,2010.Created by XMLmind XSL-FO Converter. Open Geospatial Consortium:OpenGIS Implementation Standard for Geographic information - Simple feature access - Part 2: SQLCreated by XMLmind XSL-FO Converter. option, OGC 06-104r4,2010.Created by XMLmind XSL-FO Converter.

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