The QGIS interface is divided into four interface types: menu bar, toolbars, panels, and map display. The following screenshot shows QGIS Desktop with all four interface types displayed:

The map display shows the styled data added to the QGIS project and, by default, takes up the majority of the space in QGIS Desktop. The menu bar, displayed across the top, provides access to most of QGIS Desktop's functionality. The toolbars provide quick access to QGIS Desktop functionality. The toolbars can be arranged to either float independently or dock at the top, bottom, left, or right sides of the application. The panels, such as Browser and Layers, provide a variety of functionality and can be arranged to either float independently or dock above, below, right, or left of the map display.

There are four toolbars that are particularly useful, and it is recommended that you enable them:

QGIS Desktop offers a number of customization options. You can toggle the visibility of toolbars by navigating to View | Toolbars, or by right-clicking on the menu bar or the enabled toolbar button, which will open a context menu allowing you to toggle the toolbar and panel visibility. You can assign shortcut keys to operations by navigating to Settings | Configure shortcuts. You can also change application options, such as interface language and rendering options by navigating to Settings | Options.

The QGIS Browser interface (shown in the following screenshot) is composed of three parts: toolbar, data tree view, and information panel.

The data tree view is an expandable tree listing of all geospatial data files on your computer and through connections. The information display, which takes most of the space on the application, contains four tabs that provide different views of the selected data in the data tree listing, and they are as follows:

The toolbar provides access to four functions. The Refresh function reloads the data tree view while the Manage WMS function opens the WMS management screen allowing you to manage the WMS connections. The New Shapefile function opens the new vector layer dialog allowing new shapefiles to be created. Finally, the Set layer CRS function allows you to define the coordinate reference system of the geospatial data file that is selected in the data tree view.

To load vector files, click on Add Vector Layer by navigating to Layer | Add Layer. This will open the Add Vector Layer dialog that will allow us to choose the source type and source of the dataset that we wish to load.

The source type contains four options: File, Directory, Database, and Protocol. When you choose a source type, the source interface will change to display the appropriate options. Let's take a moment to discuss what type of data these four source types can load:

To load raster data into QGIS, click on Add Raster Layer by navigating to Layer | Add Layer. This will open a file browser window and allow you to choose a GDAL-supported raster file. The commonly used raster types supported by GDAL are as follows:

To add an Oracle GeoRaster, click on Add Oracle GeoRaster Layer by navigating to Layer | Add Layer, then connect to an Oracle database to load the raster. More information about loading database layers is in the following section.

QGIS supports PostGIS, SpatiaLite, MSSQL, and Oracle databases. Regardless of the type of database you wish to load, the loading sequence is very similar. Therefore, instead of covering specific examples, the general sequence will be covered.

First, click on Add Layer under Layer and then choose the database type you wish to load. This will open a window with options for adding the data stored in a database. As an example, the following screenshot shows the window that opens when you navigate to Layer | Add Layer | Add SpatiaLite Layer:

To load data from a database, we must first create a connection to the database. To create a new connection, click on the New button to open a connection information window. Depending on the database type you are connecting to, different connection options will be shown. Once you have created a database connection, select it from the drop-down list and click on Connect; you will see a list of all layers contained within the database display. If there are a large number of tables, you can select Search options and perform a search on the database. To load a layer, select it in the list and click on Add. If you only wish to load a portion of the layer, select the layer and then click on Set Filter to open the query builder. If you set a query and then add the layer, only the filtered features will be added.

QGIS supports the loading of OGC-compliant web services such as WMS/WMTS, WCS, and WFS. Loading a web service is similar to loading a database service. In general, you will create a new server connection, connect to the server to list the available services, and add the service to the QGIS project.

Let's say that you need to make a map of the total population by county. However, the counties' GIS layer does not have population as an attribute. Instead, this data is contained in an Excel spreadsheet. It is possible to join additional tabular data to an existing attribute table.

There are two requirements, which are as follows:

To create a join, load both the GIS layer and the standalone table into QGIS Desktop. QGIS will accept a variety of standalone table file formats including Excel spreadsheets, .dbf files, and comma delimited text files. You can load this tabular data using the Add Vector Layer menu by navigating to Layer | Add Layer and setting the file type filter to All files (*) (*.*) as shown in the following screenshot:

Once the data is loaded, a join can be completed by following these steps:

Snapping is an important editing consideration. It is a specified distance (tolerance) within which vertices of one feature will automatically align with vertices of another feature. The specific snapping tolerance can be set for the whole project or per layer. The method for setting the snapping tolerance for a project varies according to the operating system, which is as follows:

In addition to setting the snapping tolerance, here the snapping mode can also be set to vertex, segment, or vertex and segment. Snapping can be set for individual layers by navigating to Settings | Snapping Options. Individual layer snapping settings will override those of the project. The following screenshot shows examples of multiple snapping option choices.

When you load spatial data layers into QGIS Desktop, they are styled with a random single symbol rendering. To change this, navigate to Layer | Properties | Style.

There are several rendering choices available from the menu in the top-left corner, which are as follows:

In the preceding screenshot, the renderer is the layer symbol. For a given symbol, you can work with the first level, which gives you the ability to change the transparency and color. You can also click on the second level, which gives you control over parameters such as fill, border, fill style, border style, join style, border width, and X/Y offsets. These parameters change depending on the geometry of your layer. You can also use this hierarchy to build symbol layers, which are styles built from several symbols that are combined vertically.

You also have many choices when styling raster data in QGIS Desktop. There is a different choice of renderers for raster datasets, which are as follows:

The following is a screenshot of the Style tab of a raster file's Layer Properties showing where the aforementioned style choices are located:

Another important consideration with raster styling is the settings that are used for contrast enhancement when rendering the data. Let's start by loading the Jemez_dem.img image and opening the Style menu under Layer Properties (shown in the figure below). This is an elevation layer and the data is being stretched on a black-to-white color ramp from the Min and Max values listed under Band rendering. By default, these values only include those that are from 2 percent to 98 percent of the estimation of the full range of values in the dataset, and cut out the outlying values. This makes rendering faster, but it is not necessarily the most accurate.

Next, we will change these settings to get a full stretch across all the data values in the raster. To do this, perform the following steps:

The blending modes allow for more sophisticated rendering between GIS layers. Historically, these tools have only been available in graphics programs and they are a fairly new addition to QGIS. Previously, only layer transparency could be controlled. There are now 13 different blending modes that are available: Normal, Lighten, Screen, Dodge, Addition, Darken, Multiply, Burn, Overlay, Soft light, Hard light, Difference, and Subtract. These are much more powerful than simple layer transparency, which can be effective but typically results in the underneath layer being washed out or dulled. With blending modes, you can create effects where the full intensity of the underlying layer is still visible. Blending mode settings can be found at the bottom of the Style menu under Layer Properties in the Layer Rendering section along with the Layer transparency slider. They are available for both vector and raster datasets.

In this example of using blending modes, we want to show vegetation data (Jemez_vegetation.tif) in combination with a hillshade image (Jemez_hillshade.img). Both data sets are loaded and the vegetation data is dragged to the top of the layer list. Vegetation is then styled with a Singleband pseudocolor renderer; you can do this by performing the following steps:

The following screenshot shows what the Style properties should look like after following the preceding steps.

At the bottom of the Style menu under Layer Properties, set the Blending mode to Multiply and the Contrast to 45 and click on Apply. The blending mode allows all the details of both the datasets to be seen. Experiment with different blending modes to see how they change the appearance of the image. The following screenshot shows an example of how blending and contrast settings can work together to make a raster 'pop' off the screen: