QGIS – How do you Snap and Trace when Digitising?

By David Crowther 

• Question:

When digitising features within QGIS how can you snap to and trace around existing features?

Answer:

Firstly, using the Digitising Toolbar, choose to make the layer editable by pressing the yellow pencil icon.

Now in your QGIS settings you will need to decide how you wish to SNAP to features. Choose the Settings > Options menu and then the Digitising Tab.

In the Snapping section you can configure:-

• how you wish to snap to features - e.g. vertex only or vertex and segments
• the snapping tolerance – the greater the value the less accurate you need to be when selecting a feature to snap to
• the snapping marker colour – which defines the colour of the icon in the map when you are hovered over a snapping feature

Next choose to open the Snapping Toolbar and select the Enable Snapping tool (magnet).

From the Digitising Toolbar choose the Add Polygon (point or line) feature so that you are r

Now hover your cursor in the map to snap to a feature to start digitising. The cursor will change colour based on the snapping marker colour (e.g. magenta) which means that you will be snapping to a feature in a chosen layer.

Now left click at the edges of the shape to snap around your chosen object.

If the object you are snapping to is complex, then you can use the TRACE tool to automatically capture the vertices, instead of manually clicking to snap. From the Snapping Toolbar choose the Enable Tracing button.

Start the new feature by choosing a vertex to snap to and then simply move the cursor around your object and the TRACE tool will automatically select the vertices between the start point and your current location. If the feature you are tracing is very complex, then choose to snap and add a vertex manually at key points around the object.

Extra Tips –

On the Snapping Toolbar, use the Edit Advanced Configuration tool to configure which layers you are able to snap to, the tolerance and the snapping options (vertex only, vertex and segment) for each of those layers.

Also, using the Offset tool on the Snapping Toolbar, you can choose to TRACE around an object and offset your new feature from the traced feature.