1. A method of obtaining on-line data useful in determining the center of area or centroid of a geometrical area of unspecified shape lying in a larger x-y scan field and having its existence indicated by a level discriminated amplitude or intensity or a parameter measurable in the z-dimension comprising: (a) scanning the x-y field in a known raster pattern and obtaining a series of discrete output voltage pulses related to elemental x-y points in the scan field and having an amplitude related to the amplitude or intensity of the said parameter; (b) generating a series of discrete clock pulses related to elemental x-y points in the scan field; (c) obtaining a series of x and y synchronizing pulses from and related to the x-y scan field; (d) establishing in relation to the clock pulses and the x and y synchronizing pulses a smaller x-y scan field in the form of a rectangular window around the geometrical area in the larger x-y scan field, the boundaries as x-y co-ordinates of the said window being known; (e) identifying, from the series of discrete clock pulses, those pulses that coincide with pulses obtained in (a) having an amplitude above a preset voltage level and which fall within the boundaries of the smaller x-y scan field such as to define sample point areas in the geometrical area; (f) making an overall count of the number of pulses identified as in (e); (g) making a count of the number of elemental x-y point each of the pulses identified in (e) are spaced from a boundary of the window in both the x and the y directions to provide x-moment arm counts and y-moment arm counts; and (h) summing the x-moment arm counts and the y-moment arm counts, said counts taken with the count obtained from (f) providing data for computing the center of area or other mathematical functions of the geometrical area.