1. A method of determining a plane in a depth image, the method comprising: dividing a portion of a depth image into a plurality of areas; fitting a two-dimensional line to depth points in each of the plurality of areas; and combining two or more of the plurality of two-dimensional lines to form a three-dimensional plane estimate, wherein the plurality of areas are greater than one pixel in each direction, and wherein the fitting the two-dimensional line further includes: dividing the area into a plurality of scan lines; selecting a depth point with a highest depth value out of a set of the depth points on each of the plurality of scan lines; discarding depth points which are not further away than the farthest depth point on an adjacent scan line of the plurality of scan lines; and fitting remaining depth points of the set of the depth points so as to calculate the two-dimensional line.