1. An analysis system for determining a value representative of a parameter of a selected feature in a field of view by dissociating from a video signal generated from the field of view that portion of the video signal representative of the selected feature, comprising: video means for imaging and line scanning in a predetermined line scan format along lines traversing a plurality of features in the field of view and for generating a video signal representative of line scan intercepts with the features, from which the value of at least one parameter of the scanned features can be derived; means to generate a position signal defining a position within the field of view of at least a single point to identify the selected feature; means responsive to the position signal and the video signal for generating a first signal by comparing a signal generated from one line of scan as a function of the position signal, with a signal generated from a subsequent line of scan as a function of the position signal, including gating means responsive to the video signal and the first signal for generating a dissociated signal representative of the intercepts of the selected feature; and means for determining the value of the parameter as a function of the dissociated signal.