Remembering in the City: Characterising Urban Change

City form conveys two images – the experiential and the remembered. The urban environment therefore is a cacophony 
of complex visual stimuli experienced with the often conflicting memory associations we attribute to them. Our appreciation of the rapidly changing built environment is therefore relative rather than absolute. In this sense the temporal and spatial components of the city merge to form our interpretation of city space. This paper presents emerging retrogressive landscape analysis, from the 
domains of landscape planning and heritage, to examine the possibility of a city-wide assessment of its potential to create ‘double exposure’ – walking simultaneously in the past and the present. This is not simply derived from our experience of individual architectural structures (urban form), but also our interpretation of past movement routes, boundaries and morphology (urban code). 
For this reason it may be necessary to look beyond a heritage which focuses on distinct ‘special’ places and protected buildings and look toward a heritage of temporal change processes and ubiquitous urban evolution; since it is also our interpretation and understanding of these which contributes to our full appreciation of city ‘character’.


Introduction
With regard to research into urban landscape in Britain, Conzen (2004) has provided greatly influential perspectives on the development of an urban morphology. His use of analogy of the 'genetic' plan in studies such as Ludlow in 1946 (Conzen 2004) underlines an early association between urban form and its evolutionary and inherited nature. Conzen considered that urban space was formed by morphological regions (spatial) and frames (temporal) consisting of three main elements -these were: morphology; building fabric; land and building utilisation. The histories of plot boundaries and their ability to reveal urban development process were part of this 'genetic plan' and formed central components of Conzenian analysis: "the past provided object lessons for the future <…> rooting the future management of the urban landscape in its historical development" (Whitehand 2001: 106). It may be considered therefore that this perspective outlines a key potential non-preservatory role for 'the past' as a means to inform and contextualise change rather than resist it.

REMEMBERING IN THE CITY: CHARACTERISING URBAN CHANGE Stephen Dobson
By considering heritage in terms of an ongoing 'inheritance' and 'ancestry' it may be perceived as evolving and in a state of flux; an integral part of a process of change. Traditionally, however, heritage values in a decision-making context have focussed more on a static vision of the past -distinct, preserved and isolated from change. It is asserted here that, whilst this distinction may provide a convenient means for dealing with heritage in practice it is an artificial one. By acknowledging that our lived experience of the city will comprise of physical, cognitive and social spaces (Lefebvre 1991) -which blend the current, the past and the imagined -it may be necessary to build more sophisticated means of actively using heritage in all aspects of city planning.
Time is regarded in much research as intrinsically embedded within our constructs of place and as formed through our lived experience: "<…> time is embedded in social, spatial and embodied experience, also involves recognising the multiple experiences of time. Burrell compares 'chronos' (clock-time) with 'kiros' Ur ba nis ti kairar chi tek tū ra OWnPLan ninGanDar chi tec tU re 1392-1630print/issn1648-3537online (subjective time)" (Dale and Burrell 2008). Hull et al. (1994) outline key literature on 'place identity' and the integral nature of time and particularly the importance of our continued ability to read its passage in the structure of our surroundings: " Lynch (1972) argues that an important function of the built environment is fixing (in bricks, mortar, steel and stone) periods of time thereby making them available for contrast and comparision to current times" (Hull et al. 1994). Hull et al. also draw upon Tuan (1980), Lowenthal (1975), Lynch (1981), Norberg-Schulz (1981) and Breakwell (1983) in underlining the relationship between our interpretation of "objects or places from our past" and our construction of 'self'. From a heritage perspective, Walsh (1992) refers to Lynch (1960) and the surveys carried out for The Image of the City in suggesting that it is not change per se that people oppose, but the rate of change: "In Los Angeles there is an impression that the fluidity of the environment and the absence of physical elements which anchor the past are exciting and disturbing <…> The interviewer remarked: 'There seems to be a bitterness or nostalgia among natives which could be resentment at the many changes, or just inability to re-orientate fast enough to keep up with them' (Walsh 1992: 152;Lynch 1960: 45).
Visualising and mapping the potential for memory associations at the city-scale however requires a shifting emphasis within heritage practice on two accounts. Firstly there is a need to shift concern from individual architectural qualities of the often transitory urban form toward a greater emphasis on the heritage values of more deeply engrained urban code. Currently in Britain, the statutory mechanisms of Listed Buildings and distinct Conservation Areas do little to facilitate this shift. Secondly there is the need to consider the potential for both form and code to contain material and spatial reminders of earlier phases so as to understand how these might merge with the newly developed urban environment within an individual's experience of space -via shared memory and association. The urban environment therefore contains demarcations of time relating to its form or 'phenotype' which convey these cycles of change but is also equally held within its genotype -i.e. the code and structure within which we move and through which our lives are 'enacted' (Sudjic 2005). The influence of both time as well as the visual experience of the 'now' combine to help us construct place and is referred to by Schofield (2008) and also vividly captured by Löfgren (2002) in the term 'doubleexposure': "We don't just experience place by seeing it, and processing information about it from a purely visual encounter" (Schofield 2008: 19). "<…> We also give landscape a history: well-trodden paths of memo-ries, myths, names, moods and smells. This often leads to double exposure in that we walk in the past and the present simultaneously." (Löfgren 2002: 42). Lefebvre (1991) examines the interplay between space perceived and space remembered: "the 'physicality' of materiality, its "thingness" and the 'imaginary' aspect of materiality, that which conveys its social, cultural and historical meaning in, for example, the meanings and memories we associate with particular objects and places" (Dale and Burrell 2008: 7).

Retrogressive Historic Landscape Mapping
Retrogressive historic analysis (Rippon 2004) of the urban landscape is a key approach to help map the cycles of change. This form of analysis is typified in England by English Heritage's programme of Historic Landscape Characterisation (http://www.english-heritage.org.uk/ professional/ research/landscapes-and-areas/characterisation/): "The aim of most HLC studies is to characterise the distinctive historic dimension of today's urban and rural environment within a given area <...> It begins with the systematic identification and description of many of the historic attributes of the contemporary rural and urban landscape <...> These attributes include aspects of the natural and built environment that have been shaped by human activity in the past -the distribution of woodland and other semi-natural habitats, the form of fields and their boundaries, the lines of roads, streets and pathways, the disposition of buildings in the towns, villages and countryside." (Clark;Darlington and Fairclough 2004: 6-7).
Most importantly, retrogressive analysis starts with recording the landscape that we see today and explores the varying degrees of time-depth which influence its current appearance. Character areas are digitised into a Geographic Information System (GIS) with associated text and information including the level to which previous phases may be visible in the present. This visibility of the past or historic legibility presents a powerful tool for the non-heritage urban professional to make use of -especially since historic legibility refers to any material evidence whether this is form or code. The HLC process therefore involves building an evidence base of past land uses and the characteristics of these over time along with our ability to read these in the present. This map resource therefore offers a means to incorporate a temporal dimension to spatial analysis for all areas, not just the distinct, and so supports a comprehensive analysis of the city. This papers explores the use of this tool to map the areas of surviving historic legibility, specifically where there has also been significant land-use changes in recent times. This combination of significant change with an inherent legibility of the previous landscape thus presents areas where there may be the potential for surviving reminders of the past to promote 'double exposure'. By mapping these we might better understand the conflicting images of the city which residents from different generations may carry with them. It is hoped that this would lead to a more refined appreciation of how urban character might be constructed through the combination of experiences.

Example: City of Sheffield, United Kingdom
The City of Sheffield in South Yorkshire (Fig. 1), England has a population of around 530,000 with an urban density of almost 4000 / km 2 . It is a post-industrial city which has seen a great deal of regeneration activities and re-development, particularly since the 1990s. The South Yorkshire Archaeology Service HLC survey for South Yorkshire (http://sytimescapes.org. uk) provides characterisation and time-depth data for the whole of the South Yorkshire area and in doing so records the whole metropolitan borough of Sheffield using approximately 2000 polygons. These define current landscape type in both broad class and subclass terms along with the date of origin for the current landscape type. Previous types are also recorded regressing back up to five previous phases all with associated dates of origin. Historic legibility of the previous type within the present is also recorded on an interval scale of four levels from 'invisible' (i.e. no evidence within form or code remains of a previous phase) to 'significant' (i.e. the previous phase is highly readable within current form or code).
When considering the character of urban areas for their potential to facilitate 'double exposure' the presence of legibility in areas which may have changed dramatically in recent times is a key attribute. Therefore, by mapping where these factors coincide it is asserted here that it is possible to build a sampling frame for urban areas within which we might consider that residents familiar with their surroundings will be reminded through interpretation of evidential qualities of the potentially conflicting images of townscape. Whilst remnant structures and reused buildings may be obvious sources of double exposure, the recording of evidence embedded in urban code as well as form (as is the case with this type of retrogressive landscape analysis) incorporates a more complex set of spatial 'triggers' for memory. These include the potential for street alignments, plot boundaries and movement routes, for example, to provide memory triggers which lie embedded within the very genotype of the city. Figure 2 illustrates results defining such an evidential sampling frame. Initially the character areas are filtered within a GIS for those character types which have changed use from previous to current type. An additional filter is applied upon the date category for Fig. 1. themetropolitanboroughofsheffieldasdefinedbyhLcpolygons.themainurban areaofthecityishighlightedinblack the current type to highlight where such changes have occurred in the 20 th century or later. The resulting map therefore (figure 2: left) provides an important means to establish how the readable urban phenotype and genotype might contribute to a multiple reading of current and previous urban landscapes. By classifying these based upon various date ranges we might also image how different generations might respond to these areas.
The additional analysis provided in figure 2 (right) applies a third filter to the original query to isolate specific previous landscape types eg. green, industrial or residential. The areas are then buffered using 500 m buffer zones to provide generalised areas of the city to which one might associate a particular kind of change. For example, the map of 'green' areas is essentially portraying where there is some kind of readable change within a currently non-green landscape of its previously green past. By comparison with figure 1 we may establish that much of this change has occurred around Sheffield's urban fringe and outline a shift to suburban housing in Sheffield through various programmes of 20 th century slum clearance. The clearance of residential properties from the city centre core can be seen in the residential double exposure map (Fig. 2: right).
The images presented here therefore are models of a remembered city based upon recorded changes and our potential to read these changes. Where there are changes from an earlier green past, for example, or alternatively an earlier industrial or residential past such information points toward the character areas of a remembered past. It is easy to forget what has preceded change; a process which therefore results in the dislocation and sense of rapid change referred to by Lynch. However, whilst reminders persevere we might consider that the act of remembering is supported. Whether the influence of the readable past on our perception of the present might be considered culturally positive or negative is a matter for communities and planners/ designers to explore together.
Magnaghi references the Greater London Plan 2002 as an example of "brusquely interrupted relations with history and memory of place"."The deconstruction of the memory and biography of the territory forces us to live in anonymous sites, reduced to supporting the functions of an instant society, which has brusquely interrupted relations with history and memory of a place" (Magnaghi 2005: 11).
The underlying message is of the importance of 'intradependence' as outlined by Theobald (1997). Where there is a declining interdependence between people and places, the local community may be unable to pass on the communal memories and meanings of place to newer community members. In this case it may be suggested that places themselves must embody the changes that they are comprised of and must be afforded the ability to communicate these through a legibility of historic change; heritage, therefore, being defined here as the inherent readability of the process of change. The need to engage with the historic character and readability of place therefore is a central component of local sustainability and in simple terms meets two crucial questions posed by Selman (1996): whether the concern has local relevance and whether it can remain durable.
The materiality of past social relations at a landscape-scale is therefore comprised of both landscape shapes, which may be mapped, but also their historic associations and their current, appropriated communal relevance -both as contemporary landscape elements and as evidence. The collaborative nature of spatial planning has the capacity to accommodate multi-stakeholder, trans-disciplinary discourse regarding a democratically constructed landscape heritage and therefore the challenge for HLC lies in facilitating this process at a wider scale than the site.

Conclusions
The interactions between us and the landscape elements, which might trigger memory or a reading of more distant, learnt and imagined pasts, is inevitably lessened over time. However this act of 'forgetting' is accentuated by unsympathetically planned change and the loss of evidence of interactions -the visible timedepth and historic legibility of place. This represents a process of dislocation between people and place. Whether describing physical remains or the historic trajectories of movement, those landscape characteristics which have persevered, despite change, embody different meanings to different groups or generations. These are characteristics of place which may become appropriated in recent times as people's lives repeatedly coincide with the eroding evidence of past activity. Physical remains, morphological regions, spaces and movement routes are therefore all characteristics of landscape which become imbued with "social stuff" (Latour 2007) -they are 'social objects'/ trajectories which are informally evolved and evolving.In an urban context, the sympathetic treatment of these persevering 'lines of life' (Cullen 2006) or 'time-marks' (Walsh 1992) therefore requires the domains of design and planning to consider these attributes in a manner which will enable them to erode at a pace which is acceptable to those for whom these may have meaning. This is outlined with reference to Lynch (1960) and Walsh (1992), highlighting that the rate of change is as important as the nature of the change itself. This paper has aimed to illustrate how retrogressive analysis, which considers all urban space as being of potential historic significance with the ability to convey a readable past in many forms, can be used to map the potential for double exposure. In doing so this work presents a sampling frame from which to conduct further studies or to simply understand a little more the conceptual spaces which may merge with the physical. This occurs through the recollections of those walking simultaneously in a present-past for which these memories are maintained and strengthened by an inherent readability of the past within the very genetic plan of the urban spaces they move between.