How do I classify contour lines on specific levels?

Question

I have a shapefile with contour lines. When I go to classify it to decimeter interval it chooses some rounded off to for example 5,8 while 5,7 is classified by 5,70000001 or simliar. How do I go about getting it round off to the decimeter?

Answer 1

Rather than directly using your "ELEV" field as classification value you may use some expression to round the value to the desired number of decimal.

In your case you could use Round("ELEV", 1)

Answer 2

As far as I know, there is no way to define the number of decimals for your legend when you symbolize with the Categorized option.

If you use the Graduated option, you get that ability.

Answer 3

You can use pyqgis to round the label values:

layer = QgsProject.instance().mapLayersByName("Random points")[0] #Create a layer object from layer name. Adjust this line
value_field = "ELEV" #Name of the field to style by. Adjust this line
renderer = layer.renderer() #Create/extract a renderer object

cats = renderer.categories() #And categories
for cat in cats: #For each category
    val = cat.value() #Extract its value
    if type(val)==float: #If it is a float number
        newlabel = f"{round(val, 1)}" #Round it and convert to a string
        cat.setLabel(newlabel) #Set the category label to it
    
new_renderer = QgsCategorizedSymbolRenderer(value_field, cats) #Create a new renderer
layer.setRenderer(new_renderer) #Apply it to the layer