What is Urban Density? How Should We Measure It?

Salon writes about the various approaches to measuring denisty:

The hardest question, in calculating residential density, is the only question: How do you measure area? Because official urban boundaries fluctuate wildly between cities — some, like Toronto or Jacksonville, include vast tracts of suburban land, some, like Atlanta or Miami, are tiny fractions of their metropolitan areas — comparative rankings based on city size are useless. The Harvard economist Ed Glaeser, for example, has written that Shanghai and Beijing are less than half as dense as Los Angeles, which tells you everything you need to know about the utility of comparative urban density measurements.

But even if we restrict the measurement to one city or neighborhood, what ought to be included? The land occupied by residential buildings, certainly. What about roads? On the one hand, they are inflexible, generally unavailable for construction purposes; on the other, their size is a prime determinant of neighborhood character, commerce, and the potential for mass transit.

Should green space count against density? Including Central Park substantially lowers the density of Upper Manhattan, producing a metric at odds with the area’s settlement patterns, transportation uses, and the distribution of shops and restaurants. Yet naming the Upper West Side the densest neighborhood in Manhattan also seems misleading, since it’s sandwiched between the vast open spaces of Central Park and the Hudson River.