Drift (Dv) expresses the distance from a subject point to the centre of gravity of its isovist (Dalton & Dalton, 2001). In an isovist field, drift identifies the inherent ‘flow’ within a series of spaces, or the ‘pull’ or ‘push’ one might feel from the volume of space itself. To move along contours low in Drift is to be in the centre of one’s isovist, and so be visible from all directions. High Drift identifies regions from which space can be surveyed with a minimum of head turning.
To determine Drift, the Isovist_App calculates ‘M’, the area weighted mean of all radial-plan intersections for every point ‘V’ in plan. It then measures the distance from M to V. The outcome is an absolute value of length.
In notation form the calculation for Drift is expressed as:
Where ‘Ri’ is the coordinates of the nearest radial intersection, ‘Li’ is radial length, ‘‘Xm and Ym’ the coordinates of ‘M’, ’Xv and Yv’ the coordinates of ‘V’, and ‘n’ the total number of radials sampled.