circle.precision()

circle.precision([angle])

If precision is specified, sets the circle precision to the specified angle in degrees, and returns this circle generator. The precision may also be specified as a function; this function will be invoked whenever a circle is generated, being passed any arguments passed to the circle generator. If precision is not specified, returns the current precision accessor, which defaults to:

function precision() {
  return 6;
}

Small circles do not follow great arcs and thus the generated polygon is only an approximation. Specifying a smaller precision angle improves the accuracy of the approximate polygon, but also increase the cost to generate and render it.