1. A method of recalibrating a depth image sensor, the depth sensor capturing image data from a 3D space, the method comprising: a) measuring a depth value of a person in the 3-D space using the depth sensor; b) determining a number of pixels in the image of the person along an axis; c) calculating a length of the person along the axis based on the depth value measured in said step (a) and the number of pixels determined in said step (b); d) receiving input from the person regarding a true length of the person along the axis; e) determining a true depth value of the person independently of the measurement of said step (a) by comparing the length calculated in said step (c) with the true length received in said step (d); f) modeling a depth error resulting from a deviation in the true depth determined in said step e and the depth measured in said step (a); and g) correcting the depth measurements made by the depth sensor using the depth error modeled in said step (f).