Toby’s latest post in his site engineering series reminded me that there’s yet another consideration in how we define slope — the audience to whom we’re communicating. When I’m discussing the pitch of a walk with another landscape architect, I’ll talk about it as a percentage (“From the wall to the driveway the walk slopes at 3 percent, with a consistent 2 percent wash”) When a contractor looks at a slope, he tends to see it as a ratio (“So I’ll set the stones to slope a quarter of an inch per foot, then”). This switching back and forth was confusing to me at first, but then I realized that the language (slope) is the same; it’s simply a matter of mastering the various dialects (ratios, percentages, decimals).
(Several years ago I had a meeting with one of the chief engineers on Boston’s Big Dig, about some grades one of the downtown tunnels. We chatted for a while, and he batted around elevation numbers with amazing facility. For me, physical slope is easier to visualize than numbers, and I still work at making the conversion from physical form to numeric form. I asked the engineer how he understood slope — as form, or as numbers — and he told me that the numbers were an easier way for him to conceive of it. He could read the slope simply from seeing the sequence of station numbers. It was remarkable to me that he could envision the nature of the grade this way, and showed yet another way of conceiving of slope.)