QGIS » QGIS Application

When you try to create a feature by tracing other features in other layers, the tracing tool will often stop working with the displayed error

Tracing : Tracing may not work correctly. Please check topology of the input layers

which also shows up in the Messages log panel. It shows up even when all layers show no errors when checked with the tools in the processing toolbox. At the same time as tracing stops working, the Qt log panel shows the line

WARNING    GEOS exception: std::exception
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When starting from a terminal

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Warning: GEOS exception: std::exception
Stacktrace (piped through c++filt):
/usr/bin/qgis.bin(+0x10714)[0x55707d3b4714]
/usr/bin/qgis.bin(+0x10c40)[0x55707d3b4c40]
/usr/lib/x86_64-linux-gnu/libQt5Core.so.5(+0xaee28)[0x7f56aba9ee28]
/usr/lib/x86_64-linux-gnu/libQt5Core.so.5(+0xaf0c8)[0x7f56aba9f0c8]
/usr/lib/x86_64-linux-gnu/libQt5Core.so.5(QMessageLogger::warning(char const*, ...) const+0xaf)[0x7f56aba7117f]
/usr/lib/libqgis_core.so.3.7.0(+0xdcc1f8)[0x7f56adaec1f8]
/usr/lib/x86_64-linux-gnu/libgeos_c.so.1(GEOSContextHandle_HS::ERROR_MESSAGE(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ...)+0xde)[0x7f56a66261be]
/usr/lib/x86_64-linux-gnu/libgeos_c.so.1(GEOSNode_r+0x98)[0x7f56a6617018]
/usr/lib/libqgis_core.so.3.7.0(QgsTracer::initGraph()+0x7b4)[0x7f56ad777312]
/usr/lib/libqgis_core.so.3.7.0(QgsTracer::init()+0x4a)[0x7f56ad77852e]
/usr/lib/libqgis_gui.so.3.7.0(QgsMapToolCapture::tracingAddVertex(QgsPointXY const&)+0x8a)[0x7f56ae9ee426]
/usr/lib/libqgis_gui.so.3.7.0(QgsMapToolCapture::addVertex(QgsPointXY const&, QgsPointLocator::Match const&)+0x252)[0x7f56ae9efe6a]
/usr/lib/libqgis_app.so.3.7.0(QgsMapToolSplitFeatures::cadCanvasReleaseEvent(QgsMapMouseEvent*)+0x1d3)[0x7f56af58a7b9]
/usr/lib/libqgis_gui.so.3.7.0(QgsMapToolAdvancedDigitizing::canvasReleaseEvent(QgsMapMouseEvent*)+0x1cf)[0x7f56ae9ec99f]
/usr/lib/libqgis_gui.so.3.7.0(QgsMapCanvas::mouseReleaseEvent(QMouseEvent*)+0x3b5)[0x7f56ae9c9ac7]
/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5(QWidget::event(QEvent*)+0x1d8)[0x7f56ac65a7d8]
/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5(QFrame::event(QEvent*)+0x1e)[0x7f56ac6fde7e]
/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5(QGraphicsView::viewportEvent(QEvent*)+0x16b)[0x7f56ac9480fb]
/usr/lib/x86_64-linux-gnu/libQt5Core.so.5(QCoreApplicationPrivate::sendThroughObjectEventFilters(QObject*, QEvent*)+0x9b)[0x7f56abc5b1ab]
/usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5(QApplicationPrivate::notify_helper(QObject*, QEvent*)+0x71)[0x7f56ac61c491]
../../src/core/qgsmessagelog.cpp:29 : (logMessage) [1909ms] 2019-03-31T12:46:59 Qt[1] GEOS exception: std::exception
../../src/core/qgstracer.cpp:566 : (initGraph) [8ms] Tracer Noding Exception: std::exception
../../src/core/qgstracer.cpp:583 : (initGraph) [0ms] tracer extract 425 ms, noding 1915 ms (call 0 ms), make 0 ms
../../src/core/qgstracer.cpp:760 : (findShortestPath) [6ms] path timing: prep 5 ms, path 0 ms
../../src/core/qgsmessagelog.cpp:29 : (logMessage) [5ms] 2019-03-31T12:57:25 Messages[1] Tracing : Tracing may not work correctly. Please check topology of the input layers.
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Sometimes you can reset the tracing tool by zooming in one level and then back out and it will work with no problems. Often, however, you have to stay zoomed in and sometimes have to zoom in multiple levels before tracing finally works again.

As you trace a section and then slide the canvas to trace further, you will eventually have slid your way out of the area shown at the original zoom level without the tracer throwing an error. One would think that once the problematic vertices were in view the problem would happen again.

Sometimes exiting and restarting QGIS will make the problem go away. Sometimes getting too frustrated, going and working on a different part of the map, and then returning days later to try again and things will work with no tracing problems at all.

Also happens in 3.6 and 3.45 in addition to master/3.7