The classification procedure usually leaves you with a small number of isolated, generally poorly classified or unclassified, pixels that are often located at the boundaries between two clearly assigned areas. They give the picture a ‘pointillist’ look that may be problematic for producing maps. In such a case, homogenising the classification by reassigning the pixels to one or the other class is desirable. Filtering techniques are also used to achieve this.
Modal filtering involves assigning an isolated pixel to the dominant class within which it lies. A 3x3-pixel mobile window is used to analyse the image for each pixel in order to determine the dominant class around the central pixel. The pixel is then assigned to this class. The major drawback of modal filtering is that it skews the picture in the dominant classes’ favour, especially at the boundaries between two classes, to the detriment of the less represented classes or those corresponding to linear structures, such as the road network, or dispersed habitats, such as scattered dwellings. In the image opposite the small details have disappeared, but the filter has also altered all of the contours of the image, causing the linear boundaries and straight angles to disappear.