d3.geoInterpolate()

d3.geoInterpolate(a, b)

Returns an interpolator function given two points a and b. Each point must be specified as a two-element array [longitude, latitude] in degrees. The returned interpolator function takes a single argument t, where t is a number ranging from 0 to 1; a value of 0 returns the point a, while a value of 1 returns the point b. Intermediate values interpolate from a to b along the great arc that passes through both a and b. If a and b are antipodes, an arbitrary great arc is chosen.