Urban form & morphology

Footprints, streets and a boundary are enough to describe a city’s form, and open data now supplies all three almost everywhere. What it does not supply is the definitions. A building is whatever the source says it is — in Den Haag, OpenStreetMap and the national register agree to the square metre because one was imported from the other, which is exactly why that agreement does not travel. A plot is whatever land unit you choose: the same buildings give a floor area ratio of 1.92 on street enclosures and 2.43 on a 250 m grid.

The street network has the same problem in a different form. Barcelona’s Eixample and Chicago’s Loop have almost identical blocks — about 17,000 m² between centrelines — yet the Eixample scores 0.61 on orientation order and Chicago 0.87, because the Eixample’s 45° grid falls across the histogram’s bins. Polygonising the streets of 4 km² of Den Haag gives 815 faces, a third of which are carriageway slivers and junction fragments rather than blocks.

The concepts below settle the elements of form, density measures, orientation, tessellation, blocks and urban extent; the how-to guides work through real footprints, cadastral parcels, street graphs and GHSL grids for Den Haag, Barcelona, Chicago, Rome and more; and the fixes are the five ways these measures go wrong while looking precise.

Learning paths run through this topic too — see the end of this page.