Using the geometry generator to create marker symbols at specified positions

[Colin Mazengarb]

Hi All,

I have run into an issue I don't understand! (QGIS 3.22).

 I was attempting to create a special marker symbol using the geometry generator that has me perplexed.

For a given marker I wanted to create two points at a specified distance from the centre. 

I was hoping this would be like using cartesian coordinates where the offset could be specified. However, when I created a point at 0,0 coordinates (units in millimetres) the point does not appear in the centre of the preview pane but in the top left side! Is this expected behaviour?

I am struggling to understand this and have not found any resource to explain.

Please see the image where I have created two points; the red one is based on the $geometry expression. The large green one is based on the make_point(0,0). I hope this makes sense. Any suggestions would be welcome. 

I should point out that my ultimate goal is to create a line from 2 points that I can then rotate based on an attribute field. I could create a SVG file as an alternative but I would prefer to improve my understanding of the symbol creation methods.