1. A method for creating a model of an object, comprising the steps of: (a) providing volumetric scan data produced by using a scanner to scan region in which at least a portion of the object is disposed; (b) processing the volumetric scan data with a computing device to create a signed distance grid of points, each point in the signed distance grid corresponding to a vertex of a volumetric cell in the volumetric scan data and being associated with a signed distance value specifying a distance from the point to a surface of the object, a sign of the signed distance value for each point indicating whether the point is interior or exterior to the object; (c) applying a unit magnitude gradient to the signed distance values, for determining an interval signed distance function representing the boundary of the object, to define a model of the object; and (d) using the signed distance value at points on the signed distance grid for which the signed distance value is known, to determine upper and lower local interval bounds for the signed distance value at other points or sets of points, where a difference between the signed distance values of two points cannot exceed the distance between the points.