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I have a contour map which I have applied a symbology to add arrows pointing 'downslope' (defined by the average of adjacent line segment directions) by editing a linetype and adding markers.

I am now looking at editing the spacing of these markers to suit the map I am looking to produce and have run into a problem where I have lots of small/ short contour 'loops' which are being given their own direction arrow - but these are cluttering up the main topography of the site.

I would like to stop the shorter contours displaying as at the scale I am likely to be working these will not be visible (but their symbology arrows unfortunately are). I have added a length field and extracted the contour lengths, and I would like to filter the displayed features by a length.

Is this possible to achieve, ideally without Python/ PYQGIS as access to this is difficult (IT Policies etc.).

I realise I could copy the layer and manually delete the shorter features but I am after a more flexible solution which I could apply to future situations/ dynamically (i.e. if I decide to alter the cutoff value).

Windows 10, QGIS 3.8.2

Symbology generation example

Example of symbols/ contours to hide/ filter

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  • 1
    Please add an image of your symbology window to show how exactly you create your arrows, as well as an image of a "problematic" area.
    – Erik
    Jan 16, 2020 at 11:10
  • Have you tried using rule based symbology?
    – ian
    Jan 16, 2020 at 11:48

1 Answer 1

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You can turn off the whole Marker line if it is too short using built in data driven symbology.

For example my data:

enter image description here

I want to remove the arrows from the smaller islands. These are all line features, so looking at the length of the line is good. I am working in British National Grid, so my units are in meters.

So to switch off the markers from all lines that are under 5km long:

enter image description here

I am using the "Enable layer" switch, and to edit the expression you click on the button next to the text.

With the following expression string:

$length > 5000

Which is pretty simple, if the length of the feature is over 5000 meters then switch the Marker line layer on, if not switch it off.

Result:

enter image description here

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