Methods
QGIS4 is a free and open-data software, which permits
among others things “to create, to edit, to visualise or to publish
information of different georeferenced layers”4. We
used the function “Clip” to trim the thermic layer and the green roof
data, in order to correspond to the city of Geneva. Indeed, we neglected
the thermic data for the whole Canton (which is composed of rural areas
in some parts). After reading the
literature[3][5][7], we considered that
the effect would be more visible in a real urban environment. Indeed, a
first reason is concerning the density of green roofs, which is higher
in the urban centre than in the neighbourhoods, and a second reason
concerns the fact that it is better to avoid rural areas, in order to
have a more homogeneous city-like domain of study. After creation of a
vector grid with a resolution of 20 metres and corresponding to the size of the city, we assigned for each grid
cell a thermal value and a Boolean variable for green roof presence with
the attribute table of QGIS and the function “Join attributes by
location”. A green roof would give to the local grid case a “1”
value, while no green roof would give a “0” value. The data of the
grid have then been used in the Geoda5 software, which
allows to easily describe patterns with different attributes (here:
thermal response and green roof presence). We used different sorts of
representations for our results, which are briefly described in the next
section and explained and shown in figures 1&2 at the end.