provided the sole source of information for mapping fissures,
irregular channels, and lava flows that were quickly
buried by snow and ice. On the lower flank and near the
base of the volcano, the multispectral capabilities of Landsat-
7 allow the flows to be distinguished based on vegetation
cover. Two examples attest to the need for both data
sets to accurately map the flows. The first is in the area
outlined by oval A in Fig. 2, which includes both the 1991–
1992 lava flow and lava or debris flows from earlier
eruptions. Using only the Landsat-7 imagery, we could
not easily distinguish the 1991–1992 flow from the others
(Fig. 2b). However, this was possible by using the SAR
image (Fig. 2) because the 1991–1992 flow is rougher than
its surroundings and therefore produces stronger backscatter.
The second example is from the area outlined by oval B
(Fig. 2), which includes both the pre-1964 and 1964 flows.
The SAR backscattering signals from the pre-1964 and
1991–1992 flows are very similar. However, the Landsat-
7 image indicates that vegetation is denser on the pre-1964
lava flow than on the 1964 flow (Fig. 2b). We conclude that
radar and optical images provide complimentary information
associated with the different characteristics of the
surface material and both are necessary to generate accurate
lava flow maps.