I'm trying to trace a new line onto another existing line layer (highlighted yellow), but the red rubber band will not stick or follow the existing line completely. Instead the red rubber band wanders around to points on the existing line layer or other layers nowhere near where I'm trying to trace. The map project was created in QGIS 2.18. Snapping on/all layers/vertex/tracing enabled, no offset. Snap tolerance 10px.
I tried this on a short simple line in another project and it worked, but not on the project layers I'm working with. Picture shows the chaos. AutoTrace plugin worked fine in 2.18.
Here's a pic to see how the rubber band is behaving.
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4 Answers
Try changing from snapping to all layers to advanced configuration and only snap to the layer that you want to trace along.
Also I use the trace tool extensively and I find that there can be sections where it doesn't like to behave itself. 'T' is the default hotkey to toggle snapping so in parts where it isn't working properly I'll just toggle it off and digitise a few vertices manually and then toggle it back on.
answered Nov 5, 2018 at 21:59
Changing the coordinate reference system appears to have solved my problem. I have been alternating between WGS84 and NAD83/south carolina for visual reasons and was trying to trace in WGS84 but switched over to NAD83/sc on a desperate hunch. If anyone has the same problem, try a recently used CRS instead of the current CRS. (I may have created line features in both!?) Snapping toolbar settings are: all layers/vertex/10 px/enable tracing. Works like the old autotrace that worked so well! Hope it helps someone else.
Thanks TeddyTedTed for replying so quickly.
I had had the same problem. I solved it by changing my coordinate reference system. I mean, I had started to work with WGS84 but after I got this problem I changed to a projected coordinate system: in my case I've been using MAGNA SIRGAS Colombia Bogotá. Thereby, trace tool works very well.
There's a couple of rules to happy tracing (not to mention happy automatic polygonisation from lines). As has been pointed out, reproject to a consistent projected CRS and use the advanced snapping to filter which layers to snap to. If you have e.g. a regular grid over your data, you really need to make sure its not traceable!
Also though, the linework in the layer you're tracing should start and stop at every intersection (so you may have to use the 'split lines' tool on your source data at t-junctions etc), and all lines should have a minimum of three vertices. Any area should be enclosed by at least two lines. This helps stop the trace algo from getting confused. You should also be careful about how far apart you attempt to lock in the trace line by clicking; the algo will always go shortest-path. Don't give it too many options.
