## 1 Introduction
### Representational Approach  
Cities produce a constant flow of heterogeneously structured data [9; 10]; to extract and communicate meaningful information from this data it has to be converted to a medium that can be understood by human beings, which is not a trivial process [11].  
From the late 1960s with the work of [PERSON] [12; 13], a systematic approach to the study of the most adequate way to represent statistical data was developed by [PERSON] [14; 15; 16; 17; 18], and the study of the aesthetic and conceptual properties of graphics as a vehicle to deliver meaningful information was studied by [PERSON] [19; 20; 21]. More recently, with the capacity of computers to store, manage and display large amounts of data, a more computer-centric approach to representing data has emerged [22; 23; 24; 25], with the focus shifting to explaining to the machine how to map concepts to graphical elements. According to the grammar of graphics [23] definition, the representation of spatial data would limit the number of \"aesthetics\" because the shape and position of the graphical elements is mapped to the location and geometry of the represented elements themselves, leaving usually just color [26], and in some cases pattern, to convey the desired information.  
The representation of geographic data [27; 28; 29] has specific challenges because it deals with different kinds of information [30] depending on the nature of the data, making the visualization of historical [31]and conceptual [32] information challenging, although the results can be appreciated as an art form for their aesthetic properties [33, 34].  
This paper tries to address some of the difficulties in the representation of urban data, specifically regarding the interaction between built reality and urban planning, and how it affects its three-dimensional morphology, the activities that take place in the city, and its evolution in time, considering specially its impact on the pedestrians' perception of the city.  
The representation of three categories of information of the building stock was explored in the case of study: spatial (the 3D volume of buildings and planning regulations), temporal (the year of construction) and conceptual (dominant use inside buildings).  
The representation of urban phenomena in three dimensions is challenging, while advances have been made in the realistic representation of cities using procedural methods [35, 36, 37, 38] or data captured from reality [39], the use of the third dimension for aesthetic purposes [40], to convey information through the abstract representation of the 3D volumes [41], or the generalization or simplification of the 3D geometry [42, 43] remains less explored.  
Visualizing the temporal component of spatial data is even more complex than the representation of three-dimensional data. It has been conceptually laid out [44, 45] and some examples of implementation in 4D cadastral data can be found to document the lifecycle of the building stock [46].  
The pedestrians' perception of the built environment, from the street level, has been described from the functional [47] and visual [48] perspectives; recently the use of geotagged images have allowed volunteers to evaluate the subjective perception of different neighborhoods [49].  
Finally, some techniques have been used recently introduced to explore complex spatial 2D data, using conditioned choropleth maps to visualize multidimensional data [50] and arrays of spatially positioned maps [51] to better display dense OD matrices.