Measuring urban footprint to understand city behavior
By: Alejandro Cantera
Tags: GIS, Urban Planning, Data Science, Maps, Open Source

First steps: What is Urban Footprint and how to measure it

The urban footprint measures the expansion of a city in the territory. Cities are great sources of cultural and economic wealth as well as an optimal environment for people to develop their lives, but they also have a great impact on the environment due to the pollution that they cause, the consumption of natural and energy resources, as well as the construction of infrastructures (transport, communication, services) that they need for their correct operation.The analysis of the urban footprint allows us to quantify the degree of impact and soil consumption, as well as helping us to know the spatial distribution of people and services in a determined metropolitan area.

For a correct Urban Footprint evaluation, it is not only important to be able to quantify its dimension, but also how the density is distributed in its interior. It is possible to identify different types of cities according to their pressure on the territory, for example the models with a large urban footprint and low density are less efficient and they increase the territory impact compared to other cities where the urban density is bigger and the extension smaller.

For this reason, in the current context of constant urban growth,the study of the Urban Footprint helps us, assuming that cities are indeterminate and unpredictable organisms, to improve the urban knowledge to guide how to design future resilience strategies.

Spanish Urban Footprint Map

Spanish territory covers a 87,013,900 hectares, of which 2,846,400 hectares are urban area, representing just 3% of the total.

The tool that we have developed in uDA allows us to calculate the extension of the Urban Footprint at the present time from the Spanish Cadastre open data and to qualify its density from the data of the National Statistics Institute (INE) and National Register. It is constructed from the urban cadastral blocks of the whole country, quantifying the growth of cities in a synchronized way with the Cadastre.

The consideration or delimitation of urban area has been taken directly from the one determined in the Cadastre Law*, which includes not only land consolidated with building and services, but also incorporates all soils for urban development (future areas where cities will grow) and areas occupied by public areas and services.

* Real Decreto Legislativo 1/2004, Ley del Catastro Inmobiliario

The main goal will be to quantify the size of the expansion of Spain’s principal metropolitan areas. For that reason,we decided to do the analysis at Functional Urban Area (FUA) scale. The choice to select this scale compared to the municipality scale is due to the fact that municipality dimension scale is sometimes variable in the Spanish territory and in many cases there is no correlation between the importance of the urban area and its municipality extension (a clear example is Barcelona, with a very small municipal size for its great importance).On the other hand, the delimitation of the FUA covers not only the extension of the main city, but also its area of influence. According to the definition of the Urban Audit:

The Functional Urban Area (FUA) consists of the city and its commuting zone. Once all cities have been defined, a commuting zone can be identified based on commuting patterns using the following steps: 1: If 15 % of employed persons living in one city work in another city, these cities are treated as connected cities. In this case, the first city is a part of the Functional Urban Area of the second city and does not have its own FUA. (…) 2: All municipalities with at least 15 % of their employed residents working in a city are identified. 3: Municipalities surrounded by a single functional area are included and non-contiguous municipalities are dropped. In Spain, there are 80 FUAs, representing a total of 1262 municipalities.