1 Spatial Modelling of the Terrestrial Environment - Georeferencial

238 Spatial Modelling of theTerrestrial Environment
tested would have been politically 1 acceptable during the time periods modelled. Finally,
it is also doubtful whether it would have been achievable in practice, given economic and
social trends within the County. Nevertheless, had the policies tested been achievable, they
would all have met commonly accepted, sustainability standards of today.
11.5.3 Emissions Impact of the Policy Scenarios
Emissions were calculated for both the reference and policy scenarios using the methodology
described in previous sections. As anticipated, high levels of traffic emissions were
found in the A14 corridor to the west of Cambridge, in Cambridge itself and in the vicinity
of Huntingdon. In a number of ‘hot spots’ levels approached or exceeded current air quality
standards. Plate 10 shows the difference in CO emissions between the scenarios superimposed
on a LANDSAT image background. Areas shaded in dark blue represent a large
decrease in emissions levels whilst those in pale blue represent a small decrease. Similarly,
pale pink represents a small increase whilst dark pink represents a major increase. Areas
of settlement are picked out in yellow.
A number of patterns are immediately apparent. Firstly, despite the very strong policy
measures, the A14 immediately to the west of Cambridge still shows an increase in emissions
levels. However, villages in the A14 Corridor show clear improvements and substantial
improvements are evident on the Huntingdon ring road, in the suburbs to the south west of
Cambridge and on many of the routes into the town. Conversely, substantial increases are
evident in the A10 corridor immediately to the North of Cambridge and around the rural
settlements between Waterbeach and Ely. The main feature of the pattern of change is that
there has been a clear reduction of emissions in the expanded dormitory villages to the west
of Cambridge and a sharp increase in the rural areas to the north. Overall, the net level of
emissions is lower.
Given this pattern of change, an attempt was next made to evaluate the impact of the policy
packages on settlement exposure. Plate 11 shows how the SHIRE 2000 GIS can be used
for visualization of the relationship between traffic emissions concentration and the pattern
of settlement. Building outlines have been taken from Ordnance Survey Landline data and
used as a basis for creating 3D models of built form by attaching height attributes to them
from airborne LiDAR imagery. The buildings data are superimposed on an ortho-photo
image of Cambridge viewed from the South West. Emissions concentrations are depicted
as an opaque cloud overlaid on the photography. Areas of low concentration are transparent
with no colour and as the concentration level increases, the overlay becomes increasingly
red and opaque. Visualization of the data in this way is extremely useful for investigation of
concentration ‘hot spots’ and assessment of their local impact on buildings and population.
However, a more quantitative approach is needed for strategic impact assessment.
As growth in both the reference and policy cases had been largely restricted to existing
development envelopes, the most straightforward way of achieving this was to compute
an index based on concentration by pixel and settlement area. The index calculated the
proportion of settlement pixels in each ward with non-zero emissions values weighted
1 Although Cambridgeshire County Council played an important role in the work described here, no suggestion is made or
inferred that the policies tested relate in any way to their actual policy objectives, views or intentions.