In biology the concept of a transect originally meant a line or a cross-section through a defined space along which there is a progression of flora and fauna could be found. But city planners, particularly those of a new urbanist persuasion, use the term to refer to a progression of land uses from the center of a city to the rural area, or vice versa.
A founder and leader of the new urbanist movement, the Duany Plater-Zyberk firm, displays a chart on which the transects of a typical wilderness to urban progression are referred to as the Natural Zone, Rural Zone, Suburban Zone, General Urban Zone, Urban Center, and Urban Core.
The theory goes that this classification system can explain and guide the continuum of natural systems into the urban environment, and of human economic systems into the rural environment. They propose that design guidelines for each transect are nearly universal.
Under such a system, an area might be designated as primarily commercial or primarily residential, or mostly large buildings or mostly small buildings, but residential land uses would not be arbitrarily and completely separated from residential land uses, for example.
In traditional zoning, mixing the residential and commercial uses is usually strictly forbidden. If the municipality is a little more contemporary, they may designate a mixed-use zoning district, but still the requirements for review often are quite stern.
But if you use the transect concept, the municipality looks at land uses as a continuum from wild lands to downtowns.
Even the most staunch proponents of this notion admit that some land uses are not neatly covered, and that often a particular land use, such as a large civic building, may be appropriate in a zone otherwise emphasizing smaller buildings. So in this sense, we could say that transect-based design guidelines are much more flexible than typical zoning.
Not only land uses, but also regulations covered by the zoning ordinance, such as minimum setbacks, are subject to this looser type of land use and building regulation. For example, the design guidelines may state that the setbacks should be relatively large or relatively small.
At a practical level, however, someone who really understands urban design or small town design must write the design guidelines. Typically the design guidelines would describe do's and don'ts of design for an individual site, including architecture, landscape, and how it all works together.
If you're still reading and seriously considering a transect or SmartCode approach for your community, it is essential that you also read about the related idea of a form-based code.
Although the SmartCode takes awhile to master, if people have been trained in zoning theory, in the end it's simpler. Needless to say, it must be skillfully written and administered. You'll need a city planning consultant unless you have a planning director with a lot of time on his or her hands.
However, despite the learning curve, the results can be spectacular in creating lively places that incorporate sound design principles and provide an understandable continuum of higher density of housing and intensity of land uses toward the center of the city, and an intelligible change in the landscape and building types as one approaches rural areas.
This approach is wonderfully appropriate if an entire metropolitan region wants to plan in a unified fashion. Usually we let suburban boundaries stand in the way of such true regional planning or a good regional governance system. But if we're smart enough to plan the entire urbanized area and hinterland as if it were in one political jurisdiction, the transect idea would be very applicable.
Because the cross-section is theoretical, it doesn't fit every situation. For example, in the American Midwest and many other places in the world where agriculture has dominated for a century or more, there may be no wilderness or natural zone.
To cite another typical situation, applying this theory to a metropolitan region bisected by a river might be quite different. An uninhabited island in the middle of the river might fit the Natural Zone, but be smack in the middle of the region geographically.
We would add that other zones may not exist in a particular environment as well, so it's up to the community whether or not you want to create a dense downtown core and whether the continuum idea really works for you. Obviously communities that are already built up right up next to the ocean or the mountains may not find this classification idea totally applicable.
We also think the transect concept is more useful in urban planning for newer and still developing areas than built-out cities where it would be jarring to roll back traditional zoning and start using a transect approach to all new and infill development.
In sum, we think this idealized progression fits many circumstances, but you have to adapt it to what is appropriate in your community.