In order to explain how I generated the graph of my bicycle commute, first I should explain how I got the data. Late in the Fall of 1998, I walked up the route, carrying a 4-foot level and a measuring tape. I held one end of the level on the road, holding it level of course, and measured the distance down to the road at the other end. I then walked along the route, counting my paces, until I found another likely spot. I tried to measure anywhere near any changes of gradient. I recorded the paces and the inches of elevation.
I rode the bus back down. The bus driver was real curious about what I was doing with this 4-foot long level on the side of the road for almost an hour. I tried to explain, but I don't think he believed me.
What I got from that, once I converted my paces to feet along the route and the inches to a ratio of incline (this is exactly the slope of the curve at each point, was a set of data points of the slope dy/dx of the curve at various points along the curve.
So, how do you convert this data to a plot? The first hassle was to convert these data points to a continuous function, reasonably approximating the slope of the road. I did this using Maple's spline function, but it caused some trouble. The problem is that the spline could not be manipulated as an ordinary function. A spline is a way to connect the dots from a data set to get a smooth approximating function. Usually, you want to use a cubic spline, which uses the best-approximating cubic through each adjacent set of three data points, making sure the slopes match up at the ends. This was too hard for Maple, with all the data points I had (54). So I used a ``linear spline'', which is really just a piecewise-linear approximation. This means I really just connected the dots with straight lines. The slope then is continuous, but it is not itself differentiable.
When I say that Maple couldn't handle it, I mean that Maple could not integrate the formulas I needed. As mentioned earlier, with help from Helmut Kahovec, Robert Israel, and others on sci.math.symbolic I was able to modify the spline definition enough to get it to work.
But, there is another problem. What I computed was the slope at points along the curve, measuring with respect to length along the path, not horizontal ( x ) displacement. So, the arclength parameter needed to be converted to the height and the length. It is a straightforward calculus problem to do this, though. If we have data for dy/dx as a function of s then we can find y as a function of s by
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||