Jökull - 01.01.2019, Side 114

Remote sensing of snow patches on Tröllaskagi Peninsula, N-Iceland
two aerial images from 1988 and 1994 are from the
same year as the satellite image and therefore used for
validation (Figure 5). In 1988 (Figure 5a), close to the
rock glacier a cloud obscures the snow beneath, while
snow in shadow areas is not detected due to lower
brightness leading to a misclassification of snow. The
cloud was manually selected and removed from the
snow patch area. In the aerial image 27% less snow is
identified due to the cloud cover and missing data in
the north of the study site. To see how single snow
patches are different in the two images, we manu-
ally selected snow patches in both images and com-
pared them. The snow patch on the ridge is about
7% smaller in the aerial image compared to the satel-
lite image, the snow patch in the north–east is 12%
smaller in the aerial image. Overall, the compari-
son is satisfying and the aerial image is preferable for
comparing the classification result. In the Landsat-
5 image from 1994 no clouds occur and most of the
snow patches in the satellite image are classified cor-
rectly. The snow around and on the rock glacier is
well classified in both images. The time difference
of the images in 1994 is more than a month, there-
fore snow coverage is about 70% smaller (in partic-
ular around the rock glacier and close to the ridge)
in the satellite image (acquired on the 20th Septem-
ber) compared to the aerial image (acquired on 10th
August). The differences between the selected snow
patches on the two images are also large. The snow
patch in the north east of the study area has completely
Figure 5. Comparison of aerial images and classified PSPs based on satellite images in the Kerling area. (a)
Here the aerial image is resampled to 30 m and (b) PSP map (L-5) of the year 1988 and (c) aerial image (30 m)
and (d) PSP map of the year 1994. – Sjálfvirk greining fanna af gervitunglamyndum borin saman við loftmyndir.
JÖKULL No. 69, 2019 113