I do not quite understand what you mean with the m or z picker tool,
however I have some Python code here in operation that returns a
"station" on a line by clicking on the line (or near it, as you won't
hit the line without snapping to segment). A "station" is a position
on the line, i.e. the start point of the line is station = 0, the end
point of the line is station = line.length().
Rational was to identify stations on street lines where certain
infrastructure like sewer covers are situated.
Will share if you are interested.
The same as Paolo Cavalini says.
It seem very useful.
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