Line perp to line
Is there a snapbased procedure to pick a point on a line and draw another line away, perp to the first line?
Comments

Yes you can. See attached animated gif: I have Polar and Strack ON. Esnaps Midpoint and Perpendicular are ON.

The perpendicular Snap ?
(I have that always on)But I think you mean a new Line that doesn't even touch the base line ?
So anywhere in space but just perpendicular to the base line ?If so I would go from the initial point, reaching and Psnap the base line
and change their length afterwards (graphically or by properties).But since V20, DUCS works for 2 Elements too.
So you can align DUCs and lock it with SHIFT first,
then snap your first point and draw the line in POLAR along the
perpendicular axis. 
Works nicely at least from V18 .
to draw a line pick a point , choose your snap, touch the line you want the snap to apply to, move the cursor until it gets a snapline.
works for both parallell and perp. 
Thanks boys  yes since asking the question the Perpbased was has been magically working for me too  have yet to explore the DUCS based solution
Howdy, Stranger!