Normalization of bi-directional effects in aerial CIR photographs to improve classification accuracy of boreal and subarctic vegetation for pollen-landscape calibration

Bi-directional effects were quantified and normalized in colour infrared (CIR) aerial images in order to produce high accuracy, high-resolution vegetation maps for the needs of pollen-landscape calibration. Three areas in northern Finland with different vegetation types were studied. The brightness variations of the most typical land cover type of each study site were quantified by examining the change of mean DN values of the reference sample areas on three separated wavelength bands in the overlapping images. Correlations between DN values and the scattering angles of reference sample areas were taken into account while normalizing bi-directional effects. The normalization was validated by classifying the images both before and after the normalization and assessing the classification accuracy. The accuracy increased on every study site after normalization. The highest increase was achieved in areas of uniform mountain birch forest. Increase was considerably lower on areas of fragmented vegetation. The resulting land cover classifications were used as landscape information in pollen-landscape calibration.