To digress just a little from the line discussion: That Parc Citroen photo puts me in mind of the Cornell Arts Quad, around which are ranged some of Cornell’s most historically and academically significant buildings. The Arts Quad is huge (obviously not what it has in common with the Parc Citroen lawns shown), and there is a several-foot (eleven feet?) grade change from east to west, along its short axis.
When I was a student there in the late 80s, and in the throes of learning how to analyze sites, I realized that the Arts Quad’s tilted plane created a perceptual wall for anyone walking along the west side, looking east and uphill. Standing at the bottom of the lawn and facing east, your eye perceives more lawn even than is actually there, because the plane is slanted rather than flat. In Parc Citroen, the tipped planes of lawn feel similarly more available to the eye from the walks along their low edge.
Using this kind of quiet grade manipulation can let you create a sense of greater green space than may really be available. Horizontal planes give you two axes — horizontal and vertical — to read, while tilted planes give you a more complex experience. I think that controlling the ground plane’s edge makes the experience more readable, as in this Halvorson-designed tilted plane at the Boston Federal Reserve Bank,
Manipulating the ground plane with a wash is a fine way to tweak how a space is perceived, and to give it more quiet complexity.
Cornell Arts Quad photo taken by Anjum and supplied courtesy of Flickr.