X013 Method for extracting demolition data. P. Hudson unpublished doctoral thesis (undertaken 2015‐2020) submitted 2020. - colouring-cities/manual GitHub Wiki
8.5 Tracking incremental development within static tissue
‘Static tissue’, as classified by Case Sheer, and discussed in Chapter 4, is characterised by residential plots of similar shape and size in which incremental development may easily occur. In this section, a simple, semi-automated method of measuring and visualising ‘plotsprawl’ in ‘static’ tissue in London (i.e. 2D extension within plots at ground floor level only) is tested, to assess whether OSMM footprints alone could provide insights of value into the scale and speed of material change occurring in London, at plot and city-scale.
A method was designed to calculate percentage increase in footprint area within plots between 2008 and 2018, for four random samples of static tissue in Greater London, two from Inner London and two from Outer London. In Figure 8.8, 2018 OSMM polygons, shown with black borders are shown overlaid onto 2008 polygons. The area coloured dark grey defines the 2008 footprint extent. The red section of the polygons represents the extension to the footprint, recorded by OS as occurring between 2008 and 2018. Extension shape and size is generated by subtracting the 2008 OSMM footprint area from the 2018 OSMM footprint area in ArcGIS. (Polygons highlighted in turquoise represent examples of full or partial footprints demolished between 2008 and 2018, which are addressed separately within Kimon Krenz’ approach to demolition tracking).
The four samples of ‘static’ tissue were randomly selected from Camden and Wandsworth in Inner London, and Waltham Forest and Croydon in Outer London, as shown in Figure 8.9. Differences in sample sizes were not seen as significant (three samples covered 800,000m2 and one 350,000m2). Once sample areas were selected, 2008 and 2018 footprints were downloaded from EDINA Digimap and the 2008 footprints subtracted from the 2018 using the Arc erase tool, and the resulting datasets, representing incremental horizontal growth in plots each sample area between 2008 and 2018, visualised. The total area of 2018 OSMM polygons was then compared, for each sample, to the new ground-floor area added between 2008 and 2018 as shown in Table 7.4.
In Figures 8.10 to 8.13, the extraordinary scale of plotsprawl occurring in London in just a decade is illustrated. This is so significant that street patterns, as shown in Figures 8.11 to 8.13, can be determined by extensions alone.
Table 8.4 shows the percentage increase in footprint area, between 2008 and 2018, for each sample.
Table 8.4 Percentage increase in footprint area in four samples, between 2008 and 2018
London sample areas | Ground floor area (m2) | No. OSMM polygons | Most common typology in sample
-- | -- | -- | --
LB Camden, Regent’s Park
The findings indicate that average percentage increase in footprint over the last decade has been around 10%.
The main limitation with this method was that it could only provide information on horizontal increase ground-floor area, which obviously offered only a partial picture of the amount of change to the material fabric actually occurring. Another issue arising (as also found by Krenz) was that, though it was assumed that dates assigned to OSMM footprints represented the actual year of construction of buildings, this was not the case. Extensions were often in fact aggregated by OS over a number of years. This meant that no change would be visible in an area, in some samples, for a five-year period and then hundreds of accretions would be found added in the subsequent year. Owing to this, caution must be taken when interpreting results, although the overall conclusion, that incremental development is occurring in London at an exceptional fast, untracked, rate is still considered valid. It is understood from Krenz that actual annual updates for OSMM are now being released.
Differences in terms of precision of the 2008 and 2018 surveys was also noticed. In the 2018 survey, houses, as shown in Figure 8.14, have small front additions, highlighted in red, which, when checked on Google Street View represent porches of semi-detached villas. However, these are original features which were not captured by the 2008 survey. This meant that the floor area represented by these porches was, incorrectly, picked up in 2008–2018 extension calculations. Further research is required to understand the range of features to which this issue may apply.
Figure 8.14 Front porches
shown in red on OSMM
(Ordnance Survey,
2015–2020)
It was also concluded that a longitudinal study comparing recent incremental change with historical incremental change was important, as well as research into change to/ relaxation of planning policies in London over the last two decades. Based on spot checking of terraced plots in the Camden and Westminster sample, using EDINA Historic Roam, it appeared that surprisingly few ground-floor garden extensions were built before the 1980s, perhaps because of policy, perceived structural difficulties in extending, or issues with cost. It is also possible that extensions to these buildings could have occurred (vertically) in the form of roofs extensions and/or basements which cannot be detected by this method.
It was concluded that, owing to the scale of plotsprawl identified, related research into implications for waste flows and related energy emissions would be worth undertaking. The test also illustrates how incremental development, despite forming an important feature of sustainable and resilience development (as discussed in Chapters 3 and 4), still needs to be carefully monitored, and calibrated. Knowledge held in urban morphology, the science of form, and building history, on change likely to occur at plot level, and its simulation using procedural models, (combined with open dashboards able to live stream planning data on small-scale change using a Larkham-style approach), are likely to become increasingly needed in future, to answer questions of how such a rate of extension can be accommodated in London over the next hundred years?’