Abstract Recent studies have established the role of urban planning policies in feeding the growth of informal settlements in Brazilian cities, through socio-spatial exclusion of low-income residents. The difficulties of reversing the exclusionary logic are due to several complex political, economic and cultural factors. We discuss here a factor very little discussed in the national literature: the mode of representation of the city, and in particular the invisibility of the informal city. We develop this argument from the case of Fortaleza, using documental analysis of urban plans and spatial analysis on the scale of the city and a neighbourhood. We apply a methodology that combines geoprocessing and information modeling to produce data. These data highlight the enormous distance between urban regulations and the actual urban form, and serve as a subsidy for the redefinition of urban rules for informal settlements through the Special Zones of Social Interest (ZEIS).This methodology revealed both, the inadequacy of current zoning codes but also discuss some potentialities of geographic information modeling to subsidize their re-definition.