Before you start working, make sure that you have copied the sample dataset to your filesystem and you have it located.

There are several ways of locating and opening a data file to open it in QGIS, but the most convenient of these is the QGIS browser:

The browser acts as a file explorer that is directly linked to QGIS, which only shows valid data files and can be used to easily add them to a QGIS project.

There are a few more things that you need to know that are related to this recipe. They are explained in the following sections.

Nonfile data sources

The browser also shows non-file data, such as remote services. Services have to be defined before they appear on the corresponding section in the browser. To add a service, right-click on the service name and select New connection.... A dialog will appear to define the service connection parameters.

As an example, try adding the following WMS service, using the WMS entry in the browser, as shown in the following screenshot:

A new entry will appear, containing the layers offered by the service, as shown in the following screenshot:

Before you start working, make sure that you have copied the sample dataset to your filesystem and that you have it located.

In the upper part of the description window, you will see a field named Provider. Provider defines the type or data origin and who takes care of reading the data and passing it to QGIS. For raster layers, you will see gdal as Provider. For most file-based vector layers, ogr will be the provider that will appear. They refer to the GDAL and OGR libraries, two open-source libraries that are used by many GIS programs to access both raster and vector data.

If the data is already loaded in QGIS, you can access the information about it in the Properties section of the layer (right-click on the layer name to select the Properties entry in the context menu). In the sections displayed in the left-hand side, select the Metadata section. You will see a box containing all the information corresponding to the layer data origin:

Functionality provided by the GDAL library, which (mentioned earlier) acts as a provider for raster layers, is also available in the Raster menu. This includes processing and data analysis methods, but it also includes the information tool that is used to describe a raster data source. You will find it by navigating to Raster | Miscellaneous | Info:

Data is read from the text file and processed to create geometries. All the fields in the table (all data in a row in the text file) are also added, including the ones used to create the geometries, as you will see by right-clicking on the layer and selecting Open attribute table, as shown in the following screenshot:

Along with the CSV file, this file may contains a CSVT file, which describes the types of the fields. This is used by QGIS to set the appropriate type for the attributes table of the layer. If the CSVT file is missing, as in our example's case, QGIS will try to figure out the type based on the values for each field.

Layers created from text files are not restricted to point files. Any geometry can be created from the text data. However, if it is not a point, instead of selecting two columns, you must place all the geometry information in a single one and enter a text representation of the geometry. QGIS uses the Well-Known Text (WKT) format, which is a text markup language for vector geometries, to describe geometries as strings. Here is an example of a very simple CSV file with line features and two attributes:

geom,id,elevation
LINESTRING(0 1, 0 2, 1 3),1,50
LINESTRING(0 -1, 0 -2, 1 -3),2,60
LINESTRING(0 1, 0 3, 5 4),3,70

From the layers contained in the KMZ file, you must select one of them. In this case, only a layer is contained in the elcontour1m.kmz file, so it is loaded automatically. The layer will be added to your QGIS project.

KMZ files are compressed files that contain a set of layers. When you select it, the OGR library will unzip the content of this file and then open the layers that it contains.

If just a single layer is contained, you will not see the layer selection dialog. QGIS will automatically open the only layer in the KMZ file.

As KMZ is not recognized as a supported format, the KMZ file will not appear in the QGIS browser. However, the browser supports zipped files, and a KMZ file is actually a zipped file with KML files inside it. Unzip it in a folder and then you will be able to use the QGIS Browser to open the layers it contains.

DXF files are read as normal vector layers although they do not have the same structure as a regular vector layer as they do not allow adding arbitrary attributes to each geometry.

The example DXF file that you opened contained just one type of geometry. DXF files can, however, contain several of them: in this case, they cannot be added to QGIS in one layer. When this happens, QGIS will ask you to select the type of geometry that you want to open.

In the sample dataset, you will find a file named CSS-SITE-CIV.dxf. Open it and you will see the following dialog:

Select one of the available geometries, and a layer will be added to your QGIS project.

NetCDF files are raster files, and they can be opened using the Add raster layer menu. Select NGMT NetCDF Grid for CDF as the file format in the file selection dialog that you will see, and select the rx5dayETCCDI_yr_MIROC5_rcp45_r2i1p1_2006-2100.nc file from the example dataset. Click on OK.

The proposed NetCDF file contains a single variable, which is opened as a regular raster layer.

A NetCDF file can contain contain multiple layers. In this case, QGIS will prompt you to select the one that you want to add from the ones contained in the specified file.

When only one layer is available, it is opened directly, as in the previously described example.

The NetCDF Browser plugin

Another way of opening NetCDF files is using the NetCDF Browser plugin. Select the Manage and install plugins... menu to open the plugin manager. Go to the Not installed section and type netcdf in the search field to filter the list of available plugins. Select the NetCDF Browser plugin and click on Install plugin to install it. Close the plugin manager.

The plugin is now installed, and you can open it by selecting NetCDF Browser in the Plugins menu:

Select the NetCDF file in the upper field. The other fields will be updated with the content of the selected file. Select a layer from the available ones and click on Add to add the layer to your QGIS project.

You will use the layer named poi_names_wake.shp in this recipe. Make sure that it is loaded in your QGIS project.

The OGR library, which is used by QGIS to read and open files, is also used to write them. Not all of the formats that are supported for reading purposes are also supported for writing purposes.

You can export even the layers that are not originally file-based to a file, such as a layer coming from a PostGIS database or a WFS connection. Just select the layer in the table of contents and proceed as just explained.

The Save as dialog allows additional configuration beyond what you have seen in the example in this recipe.

Open the elev_lid792_1m layer in your QGIS project.

The GDAL library is used to save the file. Not all formats supported for input are also supported for output, but the most common ones are supported for both operations.

The layer can be exported with a reduced extent. In the QGIS canvas, zoom to a small part of the raster layer. Then open the Save as dialog. In the Extent section, click on the Map view extent button. The bounding coordinates of the current map view will be placed in the four coordinate fields.

Enter a file path to save the file to and click on OK. A layer with a reduced extent covering only the region shown in the map view will be exported.

Open the layer named Davis_DBO_centerline.shp from the sample dataset.

The Davis_DBO_centerline.shp layer uses a CRS with feet as the unit, which makes this unsuitable for certain operations. We plan to use this layer in future recipes to calculate routes and work in metric units, so including this in a CRS that uses them is then a much better option:

Reprojecting is done by the OGR library when it saves the file because this is one of the options that it supports.

Raster layers can be reprojected in a similar way:

No previous preparation is needed. Batch conversion is not performed based on open layers but performed directly on files, so there is no need to open layers in QGIS before converting them.

The conversion is performed by an algorithm from the QGIS Processing framework. Processing algorithms can be run either as individual algorithms or, in this case, in a batch process.

Outputs of Processing algorithms can be created in all formats supported by QGIS. The format is selected using the corresponding extension in the filename and, unlike in the case of saving a single layer, does not have to be selected in a field or list. Using geojson as the extension for your output files, you tell processing that you want to generate a file in this format.

Although the algorithm saves only the selected features of the layer, if there is no selection, it will use all the layer features. This is the default behavior of all algorithms in processing. As there is no selection in the layers that you have converted, all of their features will have been used.

When converting files this way, the additional options from the Save as dialog are not available, and the default configuration values are used.

You can also convert vector layers with another more complex algorithm from the Processing Toolbox menu, which allows you to enter the configuration parameters used by the underlying OGR library that takes care of the process. It's called Export vector. Find it in the toolbox, right-click on it, and select Execute as batch process:

In this case, the output format is not controlled by the extension of the output filename as it happens with other processing algorithms according to what has been already explained.

The reprojection algorithm is a part of the Processing framework, so you can select the output format by changing the output file extension. You can use this to not only reproject a set of input layers but to also convert their format, all in a single step.

Raster layers can also be reprojected with another algorithm from the Processing Toolbox menu named Warp (reproject). These inputs are rather similar to the ones in the reprojection tool for vector layers with some additional parameters that are specific to raster layers. Select the algorithm, right-click on it, and select Execute as batch process to run it and convert a set of raster layers.

Pick a vector layer and load it up in QGIS. This step is optional, as you can pick the source layer from the filesystem in a later dialog.

QGIS converts your geometry to a format that is compatible with SpatiaLite and inserts it, along with the attribute table. Afterwards, it updates the metadata tables in SpatiaLite to register the geometry column and build the spatial index on it. These two postprocesses make the database table appear as a spatial layer to QGIS and speed up the loading of data from the table when panning and zooming.

The import dialog contained a few other features that are often useful. You can reproject data as part of the import process if you want, or you can specify the projection if QGIS didn't detect it properly. You can also name the geometry column something different than the default, geom; for example, utmz10n83 (this is normally not recommended). You can specify the character encoding of the text in the event that it's not handled correctly.

You can even use the dialog to append data to an existing table; for example, you have multiple counties with the same data structure that come as two separate files, but you want them all in one layer.

If, for some reason, the layer didn't import the way that you want, delete it and redo the import. If you delete layers, make sure to learn how to vacuum the database to recover the now empty space in the file and shrink its total size (this is not automatic).

What happens if this fails? Databases can be really picky sometimes. Here are some common issues and solutions:

If you need more advanced settings or can't get the QGIS tool to work, you may need to use the QspatiaLite Plugin (install this with Manage Python Plugins under the Plugins menu), the spatialite-gui (download this from https://www.gaia-gis.it/fossil/spatialite_gui/index) application, or the ogr2ogr command line (this comes with QGIS, which is part of OSGeo4w shell on Windows, or the terminal on Mac or Linux).

Pick a vector layer and load it in QGIS. You will also need to have a working copy of Postgres/PostGIS running, a PostGIS database created, and an account that allows table creation.

You should configure QGIS to be aware of your database and its connection parameters by creating a new database item in the PostGIS load dialog or by right-clicking on PostGIS in the Browser tab and selecting New Connection:

You can find more information about PostGIS at http://docs.qgis.org/2.8/en/docs/user_manual/working_with_vector/supported_data.html#postgis-layers.

Now that you can connect to a PostGIS database, you are ready to try importing data:

QGIS converts your geometries to a format that is compatible with PostGIS, and inserts it, along with importing the attributes. Afterwards, it updates the metadata views in PostGIS to register the geometry column and build the spatial index on it. These two post-processes make the database table appear as a spatial layer to QGIS and speed up the loading of data from the table when panning and zooming.

The options presented in the dialog are not all the options that are available. If you need more control or advanced options present, you'll likely be looking at the command-line tools: shp2pgsql (a graphical plugin for pgadmin3 is available on some platforms) and ogr2ogr. The shp2pgsql tool generally only handles shapefiles. If you have other formats, ogr2ogr can handle everything that QGIS is capable of loading. You can also use these tools to develop batch import scripts.

To import large or complicated CSV or text files, you sometimes will need to use the pgadmin3 or psql command-line interface to Postgres.

Need even more control? Then, consider scripting. OGR and Postgres both have very capable Python libraries.

Another option is using the OpenGeo Suite plugin, which has some additional options, such as allowing importing multiple layers into a single table or into one table per layer. To learn more about this, including how to install it, refer to http://qgis.boundlessgeo.com/static/docs/intro.html.

What happens if this fails? Databases can be really picky sometimes:

For more information on PostGIS installation and setup, refer to http://postgis.net/install.

For a more in-depth text on using PostGIS, there are many books available, including Packt Publishing's PostGIS Cookbook.