This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
Pretty much any slope that is relatively free from obstructions,
is steeper than about 6 to 1 and faces into the wind can be used to
foot launch a hangglider. The pilot just runs down the slope and takes
off when the air speed reaches 15 to 20 mph. Alternatively, when no
hills are available, towing by trucks, stationary winches and
ultralight aircraft allows gliders to get into the air .
Where a hangglider can be landed depends somewhat on the skill of the
pilot. An experienced pilot should be able to put a glider safely into
any flat spot bigger than about 50 by 200 ft and clear of
obstructions. This area requirement can vary somewhat, though,
depending on wind conditions and the surrounding terrain.