QGIS Trace Edit Tool
As you probably know, I love QGIS. It’s a fantastic tool that gets better on an almost daily basis.
However, as with most open source projects, the documentation is often lacking. I found this to be the case when I wanted to use the “Trace Edit” tool. It’s a plugin, and installed using the Plugin Fetch & Manage functionality of QGIS. Just search for “TraceDigitize” in the repositories, and click “Install Plugin.” I assumed that the tool would allow me to make a new feature that ‘snaps’ to an existing feature by tracing along the edge of the existing feature, without having to find and click every vertex. It turns out that this is exactly what the tool does…however I just couldn’t seem to get the tool to work! A Google search turned up nothing except for some developer chats about bugs and QGIS crashing while using the tool. Great!
After some frustration and experimentation, I got the tool to work. So, what’s the secret? First, you must have the “Snapping Options” set (Under Settings > Snapping Options) as the tool requires these settings to know what to snap to. Secondly, you must hold the Ctrl key while digitizing. No, this is not documented anywhere. Until now. If you don’t hold Ctrl, the tool works just like the normal digitizing tool.
One thing to note is that if your snapping tolerance is too small, or you go too fast as you trace, you will still end up with gaps as in the example above. Try tweaking (increasing) the snapping tolerance, or slow down a bit, and you should be fine!
Posted: October 13th, 2011 under GIS.