PostgreSQL/PostGIS: Interaction with Other Applications

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Gilbert Bruce
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1 PostgreSQL/PostGIS: Interaction with Other Applications

2 PostgreSQL/PostGIS Support While PostgreSQL and PostGIS are useful for storing and managing geospatial data and attributes associated with those data, the data stored in the database achieve their value through use in other applications. GRASS MapServer udig GeoTools, GeoServer Quantum GIS OGR Safe Software FME Cadcorp SIS IONIC RedSpider others... 2

3 Data Query/Import/Export with OGR The OGR vector data library includes support for read/write access to PostGIS enabled PostgreSQL databases. If the version of OGR on your system supports PostGIS, you will see a listing for PostgreSQL in the output of the ogrinfo --formats command. > ogrinfo --formats... -> PostgreSQL (read/write)... If your version of OGR supports PostgreSQL/PostGIS, you may use the ogrinfo and ogr2ogr commands to query a local or remote PostGIS/PostgreSQL server 3

4 Data Query with OGR The available PostGIS tables (layers) available from a specified database server may be obtained through a basic ogrinfo query. > ogrinfo -ro PG:'host=db.archaeoworld.com user=geog499 password=boguspw dbname=gnis' INFO: Open of `PG:host=db.archaeoworld.com user=geog499 password=boguspw dbname=gnis' using driver `PostgreSQL' successful. : fips55_geom Early versions of OGR did not access the geometry_columns table for information about geospatial tables, and therefore provide listings of system tables and spatial tables in the specified database. 4

5 Data Query with OGR Once the name of the layer(s) in the specified database have been determined, the summary layer attributes may be obtained using ogrinfo -s. > ogrinfo -ro -s PG:'host=db.archaeoworld.com user=geog499 password=******** dbname=gnis' fips55_geom INFO: Open of `PG:host=db.archaeoworld.com user=geog499 password=******** dbname=gnis' using driver `PostgreSQL' successful. Layer name: fips55_geom Geometry: Unknown (any) Feature Count: Extent: ( , ) - ( , ) <continued...> 5

6 Data Query with OGR More output... > Layer SRS WKT: GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84", , , AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]], UNIT["degree", , AUTHORITY["EPSG","9122"]], AUTHORITY["EPSG","4326"]] fips55_uid: Integer (0.0) feat_class: String (0.0) feat_name: String (0.0) fips_class: String (0.0) state_alpha: String (0.0) county_name: String (0.0) 6

7 Data Import with OGR OGR provides an efficient mechanism for importing data into PostgreSQL/PostGIS through the ogr2ogr command line utility. In essence, the import is accomplished through the conversion of one data format (i.e. shapefile) to another format (PostgreSQL/PostGIS) > ogr2ogr -f PostgreSQL PG:'host=db.archaeoworld.com user=geog499 dbname=geog499' -a_srs EPSG:4326 tgrcnty00shp tgrcnty0 This command imports a shapefile representing the New Mexico counties into PostgreSQL/PostGIS. To import the data, the required connection information for the database must be provided: PG:'host=db.archaeoworld.com user=geog499 dbname=geog499' 7

8 Data Import with OGR Since the provided shapefile does not have an associated.prj file (defining the projection of the dataset), the destination coordinate system must be specified in the command: -a_srs EPSG:4326 Finally, the source dataset and layer are provided to specify the exact data to be imported. tgrcnty00shp tgrcnty0 8

9 Data Import with OGR The result of this import may be checked through execution of the ogrinfo command: > ogrinfo -s PG:'host=db.archaeoworld.com user=geog499 dbname=geog499' tgrcnty0 INFO: Open of `PG:host=db.archaeoworld.com user=geog499 dbname=geog499' using driver `PostgreSQL' successful. Layer name: tgrcnty0 Geometry: Unknown (any) Feature Count: 34 Extent: ( , ) - ( , ) <continued...> 9

10 Data Import with OGR Additional output... Layer SRS WKT: GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84" , , AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]], UNIT["degree", , AUTHORITY["EPSG","9122"]], AUTHORITY["EPSG","4326"]] area: Real (13.3) perimeter: Real (13.3) tgr_: Integer (11.0) tgr_id: Integer (11.0) fipsstco: String (5.0) fips: String (3.0) county: String (66.0) 10

11 OGR Where Clauses Queries against an OGR dataset based upon PostgreSQL/ PostGIS may provide an SQL where clause that limits the information returned to those records that match the specified where clause > ogrinfo -s PG:'host=db.archaeoworld.com user=geog499 dbname=geog499' -where "county = 'Bernalillo'" tgrcnty0 INFO: Open of `PG:host=db.archaeoworld.com user=geog499 dbname=geog499' using driver `PostgreSQL' successful. Layer name: tgrcnty0 Geometry: Unknown (any) Feature Count: 1 Extent: ( , ) - ( , ) <continued...> 11

12 OGR Where Clauses Additional Output Layer SRS WKT: GEOGCS["WGS 84", DATUM["WGS_1984", SPHEROID["WGS 84", , , AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]], UNIT["degree", , AUTHORITY["EPSG","9122"]], AUTHORITY["EPSG","4326"]] area: Real (13.3) perimeter: Real (13.3) tgr_: Integer (11.0) tgr_id: Integer (11.0) fipsstco: String (5.0) fips: String (3.0) county: String (66.0) 12

13 Exporting PostGIS data with OGR The process of exporting data from PostgreSQL/PostGIS also uses ogr2ogr to essentially convert data from the PostgreSQL/ PostGIS format to another format (like shapefile or GML). > ogr2ogr -f GML tgrcnty0.gml PG:'host=db.archaeoworld.com user=geog499 dbname=geog499' -where "county = 'Bernalillo'" tgrcnty0 This command will access the tgrcnty0 table in PostgreSQL/ PostGIS, extract the record(s) that meet the specification in the where clause, retrieve those records, and output a GML document that contains the retrieved data. 13

14 Exporting PostGIS data with OGR The resulting GML file contains the following: <?xml version="1.0" encoding="utf-8"?> <ogr:featurecollection xmlns:xsi=" xsi:schemalocation=". tgrcnty0.xsd" xmlns:ogr=" xmlns=" xmlns:gml=" <gml:boundedby> <gml:box> <gml:coord><gml:x> </ gml:x><gml:y> </gml:y></gml:coord> <gml:coord><gml:x> </ gml:x><gml:y> </gml:y></gml:coord> </gml:box> </gml:boundedby> <continued...> 14

15 Exporting PostGIS data with OGR More output <gml:featuremember> <tgrcnty0 fid="0"> <area> <perimeter> <tgr_>17</tgr_> <tgr_id>16</tgr_id> 0.299</area> <fipsstco>35001</fipsstco> <fips>1 </fips> <county>bernalillo </county> 2.703</perimeter> 15

16 Exporting PostGIS data with OGR More output <ogr:geometryproperty><gml:polygon><gml:outerboundaryis><gml: LinearRing><gml:coordinates> , , , , , , , </gml:coordinates></ gml:linearring></gml:outerboundaryis></gml:polygon></ ogr:geometryproperty> </tgrcnty0> </gml:featuremember> </ogr:featurecollection> 16

17 Data Visualization and Management in GRASS GRASS has a number of tools for interacting with PostgreSQL/ PostGIS ps.postgisdb - A command that may be used to create and destroy PostGIS databases. db.connect and v.db.connect - Commands that establish a database connection and link a vector map to a database table (for attributes only) v.in.ogr - Import an OGR supported dataset into GRASS v.external - Create a read-only connection to an external OGR supported dataset Key to GRASS ability to interact with PostGIS enabled PostgreSQL databases depends upon how GRASS was built (enabling PostgreSQL support) and the formats supported by a specific installation of OGR (try ogrinfo --formats). 17

18 GRASS Data Import from PostGIS External PostgreSQL/PostGIS data may be imported into GRASS for use as a local dataset by executing the v.in.ogr command, using the correct OGR connection specification to define the database to be accessed. > v.in.ogr dsn=pg:'host=db.archaeoworld.com user=geog499 dbname=geog499' layer=tgrcnty0 output=tgrcnty0 location=geo_wgs84 This command will import the layer (table) tgrcnty0 from the class database (specified with the PG specification in the dsn), create a new location named geo_wgs84 and import the data into a layer named tgrcnty0 in that new location. 18

19 GRASS Data Import from PostGIS The imported data may be visualized by first opening GRASS using the newly created location, then entering the standard display commands: > d.mon start=x0 using default visual which is TrueColor ncolors: Graphics driver [x0] started > d.vect map=tgrcnty0 19

20 Display of External PostGIS Data in GRASS External PostgreSQL/PostGIS data may be displayed in GRASS (with other local or remote GRASS data) through use of the v.external GRASS command. This command creates a read-only linkage to the external data source and retrieves the data when requested for display or analysis. > v.external dsn=pg:'host=db.archaeoworld.com user=geog499 dbname=geog499' output=tgrcnty0_external layer=tgrcnty0 In contrast to the previously executed v.in.ogr command, this command registers an external connection to the specified database (defined by the dsn) and layer (specified by the layer parameter), and names the connection according to the output parameter. This layer may then be accessed like other GRASS vector datasets, but will be based upon the current data in the database. 20

21 Display of External PostGIS Data in GRASS A standard d.vect command results in the following: Which is the same result as displaying the data that were imported into GRASS. 21

22 Display of External PostGIS Data in GRASS Caveats: The version of GRASS on the class disks generates an error message when creating the connection and when displaying the data ERROR 1: ERROR: column "fid" does not exist While this error is not necessarily fatal, it may impact the display of some vector objects. The problem is supposed to have been addressed in the current GRASS release. 22

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