Tying visibility of features to zoom level by attribute in QGIS

Question

I know how to make the visibility of an entire layer conditional on the zoom level.

Is there a way to do the same thing for specific features? I have a road data set, where each feature has a class attribute that roughly corresponds to the "size" of a road (major highway, minor road, etc). I would like to configure things so that when zoomed out, only major roads are visible, with more detail showing as one zooms in on the map.

I guess I could split each class of roads out into a separate layer, but is there a way to do this that allows me to keep all the features in a single layer?

Answer 1

You can change the style to be rule-based (it will preserve the classes if a categorized style was previously set), and there you can control the display scale of each class/rule.

Answer 2

Get ready for the probably most useful option when it comes to symbology: data defined override

My suggestion: For each of your classes, set the line type based on your current scale, e.g. if(@map_scale <250000,'solid','no') - this either draws a continuous line, while the scale is below 1:250000, or no line, if the scale is above.

Where to enter this? Have a look at the screenshot:

Open the symbology of one of your classes, head for the line style, hit the data defined override symbol to the right of it, then go for "edit".

Naturally, this is a bit of a hassle if you have more complex line styles.