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PostgreSQL/PostGIS: PostGIS Installation & Manual Use

PostgreSQL/PostGIS: PostGIS Installation & Manual Use

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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.
PostGIS Introduction
• 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.
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);
An added PostGIS Function The Schema ID The table The field name The SRID, from spatial_ref_sys The geometry type Dimensions
Open Source Geographic Information Technologies - PostgreSQL/PostGIS - PostGIS Installation and Manual Use
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) );
An OGC Well-known Text (WKT) definition of a linex,y coord.
x,y coord. x,y coord.
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))
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)
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) );
Creation of POLYGON Geometries
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));
Creation of POLYGON Geometries
Creation of POLYGON Geometries
VALUES(3,'roof',GeomFromText('POLYGON((2 5,5.5 8,9 5,2 5))',-1));
Creation of POLYGON Geometries
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));
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:
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.
Conversion of Imported fips55 Data to PostGIS Geometries
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
Conversion of Imported fips55 Data to PostGIS Geometries
The result of the creation of this view may be seen by executing a simple select command: > SELECT * FROM v_fips55
Conversion of Imported fips55 Data to PostGIS Geometries
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
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
Conversion of Imported fips55 Data to PostGIS Geometries
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
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.
Spatial Queries in PostGIS
PostGIS supports a large number of spatial queries within the database. These include:
• Overlap
• Intersection
• Equivalence
• Proximity
PostGIS Spatial Queries - Examples
(geom,GeometryFromText('POLYGON((1 1,1 3,4 3,4 1,1 1))',-1))
PostGIS Spatial Queries - Examples
PostGIS Spatial Queries - Examples
fips55_geom WHERE feat_name = 'Albuquerque'),1)) as oneDegreeABQBuffer