Accessibility

All cartographic representation is the result of a transformation. The first of these is in the use of a projection which makes possible the translation from a spherical surface (the globe or earth)to a two dimensional surface (a flat sheet of paper or screen) for the purpose of representing real space. Into this space traditionally described as ‘geographic,’ the temporal dimension can be added, time, that separates places, generating a new transformation from physical (or formal) to functional space. In the maps presented here, the functional space reveals the structure of the transport network in terms of actual travelling time, repositioning cities accordingly, in effect reorganising the urban network.

The methodology developed by Professor C. Cauvin in Strasbourg analyses accessibility within a network, either starting from the totality of a matrix of origin-destination journey times (multi-polar, global accessibility) or for the same data relating to only one city (uni-polar, local accessibility).

The first stage consists in determining the relative positions of cities in terms of distance-time, basing the research, in the case of multi-polar variables, on multidimensional analyses of proximity that makes it possible to summarise a matrix of distances in two dimensions (converted into a series of coordinates) or, at the uni-polar scale, by utilising iso-distances, a procedure calculating the value of remoteness/closeness of the different network nodes with regard to the city in question.

The comparison between actual geographic locations and those calculated in space-time is represented by the map showing journey-time vectors superimposed in filigree on the patterns of global accessibility. This exercise can be extended to the whole surface area by use of an interpolation, making it possible to draw the transformed space – i.e. “anamorphosis.” Tight clustering indicates centralities, the network’s efficiency; the dilated mesh reveals slower times and marginalised zones. It should be noted that the values calculated in the margins have no validity.

The anamorphic representation can be enhanced cartographically by introducing blocks of colour and accessibility values. Globally, only average journey time can be represented. At the local scale (e.g. Roscoff, Exeter), the city, as point of departure or arrival, is encircled by isochrones which spread outwards to other cities within the study area. The two speeds of the network explain the stretching apart of the Channel, related to the slower time of sea crossings, as opposed to the accelerated time on land that brings urban centres linked by the road network closer together. Between cities, time reshapes geographic space. The Channel stretches vertically on the map for Roscoff (the closest English town being 6 hours away) but expands horizontally for Exeter, influenced by the length of journeys required to reach Cherbourg, Caen, Rouen, Dieppe, Zeebrugge...