QGIS provides easy-to-use yet very powerful digitizing tools. Digitizing or digitization is a process of encoding map coordinates and attributes in digital form. This allows you to create and edit vector data using various data sources such as text files, paper maps, or satellite imagery. This exercise will guide you through the basic interface of vector digitizing using QGIS.
1. Open QGIS and create a new project. In the Menu, select File ‣ New Project.
2. Open Project Properties and click the Coordinate Reference System (CRS) tab. Set the following options.
Note
On the fly CRS transformation allows you to combine various data layers with different reference system into a single map view.
1. Add the waterways.shp vector.
2. Zoom to the extent of waterways.shp. Right click on the layer, select Zoom to Layer Extent.
3. Create a suitable symbology and color scheme for the layer.
1. To load raster data, select Plugins ‣ OpenLayers plugin ‣ Add Bing Aerial layer.
We will use the Bing Aerial layer raster as our primary source for digitizing new features in the vector layer.
Before we can begin digitizing, we must set the snapping tolerance to a value that allows us an optimal editing of the vector layer geometries.
Tip
Snapping tolerance is the distance QGIS uses to search for the closest vertex and/or segment you are trying to connect when you set a new vertex or move an existing vertex. If it is not within the snap tolerance, QGIS will leave the vertex where you release the mouse button, instead of snapping it to an existing vertex and/or segment.
1. To set the snapping tolerance, select Settings ‣ Snapping options. Within the Snapping options window, activate the Enable topological editing by adding a check mark.
2. In the list of layers, add a check mark to the waterways. This activates snapping in respective layer. Set the snapping Tolerance to 10 pixels . Select Apply ‣ OK.
When you start editing the layer, new vertices will snap if it is within 10 pixels of another vertex within the current Map View.
3. Save your project.
We will now start digitizing roads.
Note
This process is called heads-up or on-screen digitizing. This is an interactive process, in which a map is created using a previously digitized or scanned information. It is called “heads-up” digitizing because the attention of the user is focused on the screen.
1. Make sure the waterways and Bing Aerial layers are visible.
2. Zoom-in to a smaller area, where the rivers on the image are visible.
3. Select the waterways layer, right-click and select Toggle Editing. Once the layer is in edit mode, additional tool buttons on the editing toolbar previously greyed-out will become available.
For each feature, you first digitize the geometry, then encode the attributes.
4. To digitize the geometry, click the Add Feature, left-click on the map area to create the first point/vertex of your new feature.
For lines and polygons, keep on left-clicking for each additional vertex you wish to capture. When you have finished adding vertices, right-click anywhere on the Map View to confirm you have finished entering the geometry of that feature.
The attribute window will appear, allowing you to enter the information for the new feature. Add the type of feature in the type field and the name of the feature in the name field.
To save your editing session, Toggle Editing and click Save.
Tip
In some cases, you will reach the edge of the Map View but you would like to continue adding new vertices. When this happens, use the arrow keys or press the spacebar while using your mouse to pan across the Map View.
The Node Tool
The Node Tool provides manipulation capabilities of feature vertices similar to CAD programs. It is possible to simply select multiple vertices at once and to move, add or delete them all together. The node tool also works with on-the-fly projection turned on and supports the topological editing feature. This tool is, unlike other tools in Quantum GIS, persistent, so when some operation is done, selection stays active for this feature and tool.
Basic operations
Start by activating the Node Tool and selecting some features by clicking on it. Red boxes appear at each vertex of this feature. Functionalities are:
The rest of the basic editing tools are explained below:
Full description of the editing tools and other advanced features available in the QGIS User’s Manual.
5. Finish editing the layer.
6. Save your project.