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 at point ‘V’, the Isovist_App finds the centroid point ‘M’ for the polygon of the isovist calculated at V. 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 Xm,Ym describe the coordinates of ‘M’ and ’Xv,Yv the coordinates of ‘V’