AutoTrace is a plugin for QGIS that allows users to digitise new features based on vertices from existing features. It essentially allows you to trace existing features.
The AutoTrace plugin is based on the traceDigitize plugin by Cédric Möri and others. AutoTrace has been kindly co-funded by the following British local councils and national park authorities: Angus Council, Brecon Beacons National Park Authority, Caerphilly County Borough Council, Ceredigion County Council, Dartmoor National Park Authority and Neath Port Talbot County Borough Council.
AutoTrace is available from the official QGIS plugin repository. To install the AutoTrace plugin in in QGIS, simply select Plugins > Manage and Install Plugins... from the main menu. The Plugin Manager dialog will appear. In its left-hand panel, select Get more. Within the Search box now type autotrace. Select the AutoTrace plugin from the list and click Install plugin.
To use AutoTrace, you need to first load layer(s) that you'd like to trace. With some layers are loaded, you will need to set sensible snapping options. In QGIS, from the main menu, select Settings > Snapping Options. Within the Snapping options dialog, select the layer(s) you wish to trace and set appropriate snapping settings (we recommend a 10-20 pixel tolerance, snapping to vertices).
Now add or create the layer you wish to digitise new features into.
To start tracing, first make your layer editable by right-clicking and selecting Toggle Editing. Select the AutoTrace tool from the digitising toolbar. In order to follow (or trace) an existing feature, you first need to snap a new vertex onto an existing feature. With a new vertex snapped to an existing feature, you can simply:
Right-click to finish digitising.
Please note: When tracing a continuous line based on multiple existing features, you will need to manually snap a new vertex at each new feature you want to trace over. For example, if tracing past a T-junctions, you will need to manually digitise a single vertex along the path you wish to continue tracing. This feature will be enhanced in Phase 2 of the plugin.