- Start QGIS.
The QGIS application appears. - Select Layer | Add SpatiaLite Layer.
The Add SpatiaLite Table(s) dialog box appears. - Click New. Browse and select a SpatiaLite database e.g. roads.sqlite. Click Open.
The selected database is added to the Databases drop down list. - If not selected, then choose the database in the Databases drop down list. Click Connect.
The list of tables is displayed. - Select roads. Click Add.
The roads layer is displayed in the map. - Select Database | SpatiaLite | QSpatiaLite.
The QspatiaLite dialog box appears. - In the SQL entry field, type in the following:
SELECT * FROM roads_network_net
WHERE nodefrom = 1
AND nodeto = 1000
Note: this queries the network graph for the shortest path between the from node #1 and the to node #1000. The node ids from the nodes table. - Click Run SQL.
The result is displayed.
Note: only the first record has a geometry. This geometry object contains the geometry of the shortest path. The subsequent rows list out each segment of the shortest path. - In the SQL entry field, type in the following:
CREATE TABLE route1 AS
SELECT * FROM roads_network_net
WHERE nodefrom = 1
AND nodeto = 1000
AND geometry IS NOT NULL
Note: this will create a route1 table that can be displayed in QGIS. - Click Run SQL.
A table named 'route1' is created. - In the SQL Entry field, type in the following:
SELECT RecoverGeometryColumn('route1','Geometry',4326,'LINESTRING','XY')
Note: change 4326 to the appropriate SRID of your data. - Click Run SQL.
The 'route1' table is registered as a spatial table. - In the list of tables of the QSpatiaLite dialog box, right click on the newly created table 'route1'.
A pop up menu appears. - Choose Load in QGIS.
The shortest path result layer is loaded in the map.
Monday, September 30, 2013
Routing analysis using Quantum GIS and SpatiaLite
After preparing a network graph in a SpatiaLite database for routing analysis in my previous post, you can perform shortest path analysis just by typing in SQL commands in the SpatiaLite-gui. The results are returned as result set rows which are textual - functional but difficult to visualize. It is more fun to do it in a visual environment such as Quantum GIS or QGIS for short as shown below.
Labels:
QGIS,
Spatial analysis,
SpatiaLite
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