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General description: I am using QGIS as quick visualization tool, where I make a "PostGIS" database layer and I see user generated events. Table structure as follows:

Id   |   time_of_event  | position         | heading  |
int  |   timestamp      | geometry(Point ) | smallint |

Specific problem: When I draw the position I draw it as point, I would like to draw it like so, if heading is = 0 or > 360, otherwise I would like to draw it as arrow.

I have read How to draw line pointing in compass bearing direction from point shapefile and Scaling vector field marker arrow based on attribute? but both of them are static.
And I couldn't figure out how to draw them based on a conditional, is this even possible?

P.S. I am using QGIS 3.2

Edit: so I made it display the proper direction: enter image description here

However how can I make a conditional, when heading is null, display red dot?

  • 1
    use Rule based classification; e.g. define a rule for heading = 0 OR heading = 360 and another for heading > 0 AND heading < 360 with respective symbols. rotation can then be applied separately. – ThingumaBob Sep 18 '18 at 9:42
  • @ThingumaBob very stupid question, however in which menu do I find this "rule based classification"? – thefolenangel Sep 18 '18 at 11:17
  • ,) I added all this as an answer. – ThingumaBob Sep 18 '18 at 17:18
4

Use the Rule-based classification for this.

Assuming dir is the field with the angle:

  • in the layers Symbology tab, choose Rule-based in the uppermost dropdown menu:

    ![enter image description here

    enter image description here


    • double-click the first entry to define the rule and name it if you like.
      then enter "dir" = 0 OR "dir" = 360 (or "dir" IN (0, 360)) in the Filter field (alternatively, you can open the Expression builder dialog to a GUI for function definitions):

      enter image description here

    • in the same menu, define your point symbol as you like


    • back in the rule overview, add a second rule (hit the green +) and define it:
      use either "dir" > 0 AND "dir" < 360 or select the Else bullet to simply select all features not in the other rules:

      enter image description here

    • this time, define your arrow and specify the dir field values in the Data defined override menu for Rotation:

      enter image description here


  • neatly done
  • Thank you , this is kindly simplified for "dummies", exactly what I needed. I marked it as the accepted answer! – thefolenangel Sep 19 '18 at 7:58
  • @thefolenangel glad it helped! ...I used to write those things more compact, and I hate screenshots and the hussle with them being right on spot. but then, there's no better way to ensure reproducibility. ...and since you asked...,) – ThingumaBob Sep 19 '18 at 8:03

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