QGIS Marker Lines (every N map units) shifting as I pan the map

Question

I'm drawing a map of the West Highland Way, a long-distance hiking trail in Scotland.

I want to place markers every 1 km and every 1 mile on either side of the line.

I'm using QGIS 2.18.0

Successfully downloaded the trail from OSM using the QuickOSM plugin - really easy in this case, as it's a single feature.

To tidy up the geometry, I used the Join Multiple Lines plugin to make sure all the lines point in the same direction.

I normally use QChainage plugin for this but thought I'd try Marker Lines style, using "mark every ... map units" (I'm using EPSG:27000 for this line, OTF is also on EPSG:27700)

I've added a marker line style to confirm that the lines are joined end-to-end in a consistent direction.

Here's a composite image showing the problem. Note how the kilometer markers have shifted after panning.

Can anyone explain this?

I'm not sure if a bug, or a consequence of how things are rendered (perhaps QGIS doesn't render the whole line, and is starting the 'chainage' somewhere closer to the start of the canvas extent?)

It doesn't seem to happen in Print Composer (possibly because the whole feature is rendered in one go?)

Answer 1

In the symbol "advanced" drop down menu, check the setting for whether the geometries are clipped to the visible map extent. By default qgis clips geometry to just outside the visible area to speed up the render, but this can affect marker placement for styles such as you've described.