"Roelofs used an instrument, which may be called curious for a landscape artist: a compass. With this he first measured very accurately where the horizon had to find its location [in the painting]. Then a line was drawn to determine its place. The air was therefore not started at the top.. .From the horizon the painting was alternately painted above and below the horizon, to maintain proportionality, balance and harmony; air and landscape were then developed from that core. (translation from original Dutch: Fons Heijnsbroek)"