PostgreSQL/PostGIS: PostGIS Installation & Manual Use

PostgreSQL/PostGIS:PostGIS Installation &

Manual Use

Open Source Geographic Information Technologies - PostgreSQL/PostGIS - PostGIS Installation and Manual Use

PostGIS Introduction

PostGIS represents an enhancement of PostgreSQL through the creation of new database objects and functions that enable OGC Simple Features support within a PostgreSQL database. Adding the PostGIS enhancements to PostgreSQL consists of several steps:

• If PostgreSQL was compiled from source: compile the PostGIS source code within the ‘contrib’ directory of the PostgreSQL source tree

• Changes to the compilation instructions (the ‘makefile’) must be made to enable support for PROJ4 and GEOS

• If PostgreSQL was installed as a binary, install the PostGIS binary that is compatible with the installed PostgreSQL binary.

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PostGIS Introduction

The installation process for PostGIS also requires that:

• The PL/pgSQL procedural language be enabled for the PostGIS database

• The PostGIS object and function definitions be loaded into the database

• These object and function definitions are provided as an SQL file (lwpostgis.sql) that may be executed in the database to add the required objects and functions. Once added, they are available from that point on within the so-enabled database.

• An optional, but very useful, step, is to import the provided EPSG coordinate system identifiers into a table within the database. This is accomplished through execution of the provided SQL file (spatial_ref_sys.sql) within the database.

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Creating GIS Objects

The creation of GIS objects in a PostGIS-enabled database requires that a table with a geometry column already exist for data to be imported into. This is typically accomplished through the execution of two SQL commands:> CREATE TABLE gtest (

id int4, name varchar (20) );

> SELECT AddGeometryColumn( '', 'gtest', 'geom', -1, 'LINESTRING', 2);

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An added PostGIS FunctionThe Schema ID The tableThe field nameThe SRID, from spatial_ref_sysThe geometry typeDimensions


Creating GIS Objects

Once the table to which GIS objects (attributes and associated geometries) may be added has been created, new records representing those objects may be added using the standard SQL insert command> INSERT INTO gtest (

id, name, geom) VALUES ( 1, 'First Geometry', GeomFromText( 'LINESTRING( 2 3, 4 5, 6 5, 7 8)’, -1) );

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An added PostGIS Function

SRID

An OGC Well-known Text (WKT) definition of a linex,y coord.

x,y coord.x,y coord.

x,y coord. and end of LINESTRING function


Geometries - WKT & WKB

When working with the OGC Simple Features Specification, you encounter two methods for representing and storing geometries

• Well Known Text (WKT), for example

• Point: POINT(0,0)

• Linestring: LINESTRING(0 0, 1 1, 2 2)

• Polygon: POLYGON((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1))

• Multipoint: MULTIPOINT(0 0,1 1)

• Multilinestring: MULTILINESTRING((0 0,1 1,2 2),(2 3,3 2,5 4))

• Multipolygon: MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1)),((1 2,4 5,7 8,1 2)))

• Geometry Collection: GEOMETRYCOLLECTION(POINT(0,0), LINESTRING(0 0, 1 1, 2 2))

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Geometries - WKT & WKB

While the WKT specification is frequently used for entering geometries into the database, the Well-known Binary (WKB) format is preferentially used for storage due to its greater efficiency for complex geometries.

The current definition of the WKT and WKB specifications can be found in the Open Geospatial Consortium’s set of specification documents:

OpenGIS® Implementation Specification for Geographic Information - Simple feature access - Part 1: Common architecture (OGC 05-126)

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Creation of POLYGON Geometries

The following is an extended example of the creation of a new table that contains POLYGON geometry types and associated attributes.

First, the table and associated entry in the geometry_columns table must be created> CREATE TABLE gtest_poly ( id int4, name varchar

(20) );

> SELECT AddGeometryColumn('','gtest_poly','geom',-1,'POLYGON',2);

> ALTER TABLE gtest_poly ADD CONSTRAINT id_pkey PRIMARY KEY (id);

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After the destination table has been created, records may be added to it:> INSERT INTO gtest_poly (id,name,geom)

VALUES(1,'house',GeomFromText('POLYGON((3 1,3 6,8 6,8 1,3 1),(4 4,4 5,5 5,5 4,4 4),(6 4,6 5,7 5,7 4,6 4))',-1));

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Another record:> INSERT INTO gtest_poly (id,name,geom)

VALUES(2,'door',GeomFromText('POLYGON((5 1,5 3,6 3,6 1,5 1))',-1));

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VALUES(3,'roof',GeomFromText('POLYGON((2 5,5.5 8,9 5,2 5))',-1));

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VALUES(4,'plane',GeomFromText('POLYGON((2 8.33,2 8,2.25 7.66,2.75 7.66,3.25 7,3.5 7,3.25 7.66,3.75 7.66,4 8,4 8.66,3.75 8.66,3.75 8.33,3.25 8.33,2.75 9,2.25 9,2.75 8.33,2 8.33),(2.1 8.1,2.25 8.1,2.25 8.23,2.1 8.23,2.1 8.1),(2.5 8,2.6 8,2.6 8.1,2.5 8.1,2.5 8),(2.7 8,2.8 8,2.8 8.1,2.7 8.1,2.7 8),(2.9 8,3 8,3 8.1,2.9 8.1,2.9 8),(3.1 8,3.2 8,3.2 8.1,3.1 8.1,3.1 8),(3.3 8,3.4 8,3.4 8.1,3.3 8.1,3.3 8))',-1));

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Creating Geometries from Imported Data

PostGIS tables containing geometries may be created from previously imported data through the use of PostGIS functions and select statements.

For example, the fips55 dataset, a collection of over 158,000 classified placenames with associated coordinate information may be imported into PostgreSQL to produce the following table:

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Conversion of Imported fips55 Data to PostGIS Geometries

The problem with the imported data, is that the provided geographic data are not consistent with the OGC Simple Features specification, therefore limiting the analyses that can be performed.

• Standard query methods may be used to determine locations using simple rectangular coordinates, but more sophisticated spatial queries are not supported

• The spatial data in the original table are not usable in some visualization applications that support the OGC Simple Features specification

Just as we used PostGIS functions in creating new records, the same functions may be used to create a new view of the original data that is compliant with the OGC specification.

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The first step in the conversion process consists of creating a view based upon the original data that recasts the point coordinates as an OGC WKT POINT type.> CREATE VIEW v_fips55 AS

SELECT uid as fips55_uid, feat_class, fips_class, state_alpha, county_name, GeometryFromText( 'POINT('|| to_char(primary_lon,'9999D999')|| ' ' || to_char(primary_lat, '999D999')|| ')',4326) as geom FROM fips55

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The result of the creation of this view may be seen by executing a simple select command:> SELECT * FROM v_fips55

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Depending upon the tools that will be used to access the database, you may use the view by adding an entry to the geometry_columns table for the view:> INSERT INTO geometry_columns

{‘’,’public’,’v_fips55’,’geom’,2,4326,GEOMETRY}

This command essentially registers the geom column of your view as the column that contains the geometry data for querying and rendering.

For some applications, this is sufficient. In some situations you will need to create a new table as opposed to the virtual table represented by the view. This is accomplished with:> select * into fips55_geom from v_fips55

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To optimize the performance of your newly created database, you should create at least two indexed.

A primary key for the new table, > alter table fips55_geom add constraint

fips55_geom_pkey PRIMARY KEY (fips55_uid);

and a spatial index for the geometry column:> CREATE INDEX fips55_geom_geom_idx ON fips55_geom

USING GIST ( geom GIST_GEOMETRY_OPS )

The primary key will facilitate communication of geometries and attributes to other applications that access data in PostGIS, while the spatial index will greatly increase the speed of spatial queries made against the table.

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Spatial Queries in PostGIS

PostGIS supports a large number of spatial queries within the database. These include:

• Overlap

• Intersection

• Equivalence

• Proximity

• Distance and Area

• Containment

• Buffer

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PostGIS Spatial Queries - Examples

Intersection/Overlaps (both yield the same result)> SELECT id,name FROM gtest_poly WHERE Intersects

(geom,GeometryFromText('POLYGON((1 1,1 3,4 3,4 1,1 1))',-1))

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Within> SELECT id,name FROM gtest_poly WHERE Within(geom,

(SELECT geom FROM gtest_poly WHERE name = 'house'))

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Buffer> SELECT AsText(buffer((SELECT geom FROM

fips55_geom WHERE feat_name = 'Albuquerque'),1)) as oneDegreeABQBuffer

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POLYGON((-105.651 35.084,-105.670214719596 34.8889096779894,-105.727120467484 34.7013165676453,-105.819530387688 34.5284297669944,-105.943893218798 34.3768932188293,-106.095429766957 34.252530387713,-106.268316567604 34.1601204675016,-106.455909677945 34.1032147196044,-106.650999999955 34.084,-106.846090321967 34.1032147195869,-107.033683432313 34.1601204674672,-107.206570232968 34.2525303876632,-107.358106781139 34.3768932187658,-107.482469612262 34.5284297669197,-107.574879532481 34.7013165675623,-107.631785280387 34.8889096779013,-107.651 35.0839999999102,-107.631785280422 35.2790903219226,-107.57487953255 35.4666834322718,-107.482469612362 35.6395702329309,-107.358106781266 35.7911067811072,-107.206570233118 35.9154696122371,-107.033683432479 36.007879532464,-106.846090322143 36.0647852803781,-106.651000000135 36.084,-106.455909678121 36.0647852804306,-106.26831656777 36.0078795325671,-106.095429767106 35.9154696123867,-105.943893218925 35.7911067812977,-105.819530387788 35.6395702331549,-105.727120467553 35.4666834325206,-105.670214719631 35.2790903221868,-105.651 35.0840000001796,-105.651 35.084))

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PostgreSQL/PostGIS: PostGIS Installation & Manual Use