Supported Data Formats¶

Loading a Shapefile¶

To load a shapefile, start QGIS and click on the ^{Add Vector Layer} toolbar button or simply type Ctrl+Shift+V. This will bring up a new window (see figure_vector_1).

Figure Vector 1:

Add Vector Layer Dialog

From the available options check File. Click on button [Browse]. That will bring up a standard open file dialog (see figure_vector_2) which allows you to navigate the file system and load a shapefile or other supported data source. The selection box Filter allows you to preselect some OGR supported file formats.

You can also select the Encoding type for the shapefile if desired.

Figure Vector 2:

Open an OGR Supported Vector Layer Dialog

Selecting a shapefile from the list and clicking [Open] loads it into QGIS. Figure_vector_3 shows QGIS after loading the alaska.shp file.

Figure Vector 3:

QGIS with Shapefile of Alaska loaded

Once loaded, you can zoom around the shapefile using the map navigation tools. To change the style of a layer, open the Layer Properties dialog by double clicking on the layer name or by right-clicking on the name in the legend and choosing Properties from the popup menu. See Section Style Tab for more information on setting symbology of vector layers.

Improving Performance Shape¶

To improve the performance of drawing a shapefile, you can create a spatial index. A spatial index will improve the speed of both zooming and panning. Spatial indexes used by QGIS have a .qix extension.

Use these steps to create the index:

• Load a shapefile.
• Open the Layer Properties dialog by double-clicking on the shapefile name in the legend or by right-clicking and choosing Properties from the popup menu.
• In the tab General click the [Create Spatial Index] button.

Problem loading a shape .prj file¶

If you load a shapefile with .prj file and QGIS is not able to read the coordinate reference system from that file, you have to define the proper projection manually within the General tab of the Layer Properties dialog of the layer. This is due to the fact, that .prj files often do not provide the complete projection parameters, as used in QGIS and listed in the CRS dialog.

For that reason, if you create a new shapefile with QGIS, two different projection files are created. A .prj file with limited projection parameters, compatible with ESRI software, and a .qpj file, providing the complete parameters of the used CRS. Whenever QGIS finds a .qpj file, it will be used instead of the .prj.

Creating a stored Connection¶

The first time you use a PostGIS data source, you must create a connection to the PostgreSQL database that contains the data. Begin by clicking on the ^{Add PostGIS Layer} toolbar button, selecting the Add PostGIS Layer... option from the Layer menu or typing Ctrl+Shift+D. You can also open the Add Vector Layer dialog and select Database. The Add PostGIS Table(s) dialog will be displayed. To access the connection manager, click on the [New] button to display the Create a New PostGIS Connection dialog. The parameters required for a connection are:

• Name: A name for this connection. Can be the same as Database
• Service: Service parameter to be used alternatively to hostname/port (and potentially database). This can be defined in pg_service.conf
• Host: Name of the database host. This must be a resolvable host name the same as would be used to open a telnet connection or ping the host. If the database is on the same computer as QGIS, simply enter ‘localhost’ here.
• Port: Port number the PostgreSQL database server listens on. The default port is 5432.
• Database: Name of the database.
• SSL mode: How the SSL connection will be negotiated with the server. Note that massive speedups in PostGIS layer rendering can be achieved by disabling SSL in the connection editor. Following options are available:
  • disable: only try an unencrypted SSL connection
  • allow: try a non-SSL connection, if that fails, try an SSL connection
  • prefer (the default): try an SSL connection, if that fails, try a non-SSL connection;
  • require: only try an SSL connection.
• Username: User name used to login to the database.
• Password: Password used with Username to connect to the database.

Optional you can activate following checkboxes:

• Save Username
• Save Password
• Only look in the geometry_columns table
• Only look in the ‘public’ schema
• Also list tables with no geometry
• Use estimated table metadata

Once all parameters and options are set, you can test the connection by clicking on the [Test Connect] button.

Loading a PostGIS Layer¶

Once you have one or more connections defined, you can load layers from the PostgreSQL database. Of course this requires having data in PostgreSQL. See Section Importing Data into PostgreSQL for a discussion on importing data into the database.

To load a layer from PostGIS, perform the following steps:

• If the Add PostGIS layers dialog is not already open, click on the ^{Add PostGIS Layer} toolbar button.
• Choose the connection from the drop-down list and click [Connect].
• Select or unselect Also list tables with no geometry
• Optionally use some Search Options to define which features to load from the layer or use the [Build query] button to start the Query builder dialog.
• Find the layer(s) you wish to add in the list of available layers.
• Select it by clicking on it. You can select multiple layers by holding down the Shift key while clicking. See Section Query Builder for information on using the PostgreSQL Query Builder to further define the layer.
• Click on the [Add] button to add the layer to the map.

Some details about PostgreSQL layers¶

This section contains some details on how QGIS accesses PostgreSQL layers. Most of the time QGIS should simply provide you with a list of database tables that can be loaded, and load them on request. However, if you have trouble loading a PostgreSQL table into QGIS, the information below may help you understand any QGIS messages and give you direction on changing the PostgreSQL table or view definition to allow QGIS to load it.

QGIS requires that PostgreSQL layers contain a column that can be used as a unique key for the layer. For tables this usually means that the table needs a primary key, or a column with a unique constraint on it. In QGIS, this column needs to be of type int4 (an integer of size 4 bytes). Alternatively the ctid column can be used as primary key. If a table lacks these items, the oid column will be used instead. Performance will be improved if the column is indexed (note that primary keys are automatically indexed in PostgreSQL).

If the PostgreSQL layer is a view, the same requirement exists, but views do not have primary keys or columns with unique constraints on them. In this case QGIS will try to find a column in the view that is derived from a suitable table column. It does this by parsing the view definition SQL. However there are several aspects of SQL that QGIS ignores these include the use of table aliases and columns that are generated by SQL functions.

If a suitable column cannot be found, QGIS will not load the layer. If this occurs, the solution is to alter the view so that it does include a suitable column (a type of int4 and either a primary key or with a unique constraint, preferably indexed).

SPIT Plugin¶

QGIS comes with a core plugin named ^SPIT (Shapefile to PostGIS Import Tool). SPIT can be used to load multiple shapefiles at one time and includes support for schemas. See Section SPIT Plugin for more information.

shp2pgsql¶

PostGIS includes a utility called shp2pgsql that can be used to import shapefiles into a PostGIS enabled database. For example, to import a shapefile named lakes.shp into a PostgreSQL database named gis_data, use the following command:

shp2pgsql -s 2964 lakes.shp lakes_new | psql gis_data

This creates a new layer named lakes_new in the gis_data database. The new layer will have a spatial reference identifier (SRID) of 2964. See Section Working with Projections for more information on spatial reference systems and projections.

ogr2ogr¶

Beside shp2pgsql and SPIT there is another tool for feeding geodata in PostGIS: ogr2ogr. This is part of your GDAL installation.

To import a shapefile into PostGIS, do the following:

ogr2ogr -f "PostgreSQL" PG:"dbname=postgis host=myhost.de user=postgres \
password=topsecret" alaska.shp

This will import the shapefile alaska.shp into the PostGIS-database postgis using the user postgres with the password topsecret on host server myhost.de.

Note that OGR must be built with PostgreSQL to support PostGIS. You can see this by typing

ogrinfo --formats | grep -i post

If you like to use PostgreSQL’s COPY -command instead of the default INSERT INTO method you can export the following environment-variable (at least available on and ):

export PG_USE_COPY=YES

ogr2ogr does not create spatial indexes like shp2pgsl does. You need to create them manually using the normal SQL-command CREATE INDEX afterwards as an extra step (as described in the next section Improving Performance).

Improving Performance¶

Retrieving features from a PostgreSQL database can be time consuming, especially over a network. You can improve the drawing performance of PostgreSQL layers by ensuring that a PostGIS spatial index exists on each layer in the database. PostGIS supports creation of a GiST (Generalized Search Tree) index to speed up spatial searches of the data (GiST index information is taken from the PostGIS documentation available at http://postgis.refractions.net).

The syntax for creating a GiST index is:

CREATE INDEX [indexname] ON [tablename]
  USING GIST ( [geometryfield] GIST_GEOMETRY_OPS );

Note that for large tables, creating the index can take a long time. Once the index is created, you should perform a VACUUM ANALYZE. See the PostGIS documentation (POSTGIS-PROJECT Literature and Web References) for more information.

The following is an example of creating a GiST index:

[email protected]:~/current$ psql gis_data
Welcome to psql 8.3.0, the PostgreSQL interactive terminal.

Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help with psql commands
       \g or terminate with semicolon to execute query
       \q to quit

gis_data=# CREATE INDEX sidx_alaska_lakes ON alaska_lakes
gis_data-# USING GIST (the_geom GIST_GEOMETRY_OPS);
CREATE INDEX
gis_data=# VACUUM ANALYZE alaska_lakes;
VACUUM
gis_data=# \q
[email protected]:~/current$

Usage¶

• Import data to PostGIS (Importing Data into PostgreSQL) using for example the PostGIS Manager plugin or the SPIT plugin

• Use the PostGIS command line interface to issue the following command (this is an example where “TABLE” is the actual name of your PostGIS table)

  gis_data=# update TABLE set the_geom=ST_Shift_Longitude(the_geom);

• If everything went right you should receive a confirmation about the number of features that were updated, then you’ll be able to load the map and see the difference (Figure_vector_5)

Creating a new SpatiaLite layer¶

If you want to create a new SpatiaLite layer, please refer to section Creating a new SpatiaLite layer.