counts 
Fig. 1 
Frequency distribution of sinkholes of various sizes 
perpendicular ( ) and parallel ( ) to the topographic 
contours. Frequency is given in the number of intercepts 
(counts) and size in picture points of 125 measured in the image. 
OPTICAL FILTERING AND THE ORIENTATION OF LAND FORMS 
Optical filtering can be used for the analysis of patterns of linear features showing 
distinct orientation in one or more directions and/or for the study of the spatial 
frequency (distance or spacing) of objects. When an aerial photograph is placed in 
the narrow parallel beam of a coherent laser light a Fraunhofer diffraction pattern 
will be formed because the photographic plane can be considered to form a grating, 
every point/slit of which acts as a secondary source of emitted light. This light 
will interfere with the light emitted by the other secondary sources of light in ac 
cordance with the principle of Huygens. The two-dimensional diffraction pattern 
is a so-called Fourier transform and would be produced at infinity if it were not 
carried by means of a system of lenses to a set focal distance where certain ap 
propriate filters can be placed. 
The spectrum is always symmetrical with respect to its central point which is 
formed by the non-diffracted portion of the light. The filtering techniques allow 
for the elimination of certain directions/azimuths or, inversely, for the elimi 
nation of all directions except the one(s) in which certain features of particular 
interest stretch. Other filters are used for filtering according to spatial frequency.