1. A method of zooming and panning an image on a computing device having a touch sensor, comprising: detecting a first plurality of touch points with the touch sensor, each touch point comprising a respective location of a corresponding user contact in a first set of user contacts; identifying a first centroid of the locations of the first plurality of touch points; identifying a first mean absolute deviation value of the locations of the first plurality of touch points; after identifying the first centroid of the locations of the first plurality of touch points, detecting a second plurality of touch points indicative of movement of the user contacts; identifying a second centroid of the locations of the second plurality of touch points, wherein the second centroid is in a different location than the first centroid; identifying a second mean absolute deviation value of the locations of the second plurality of touch points; determining a scaling factor comprising a ratio of the second mean absolute value to the first mean absolute value; determining a pan gesture corresponding to a direction and a magnitude of translation from the first centroid to the second centroid; scaling the image based on said scaling factor; and panning the image displayed on the computing device based on said determined pan gesture.