Monitoring using Optical Sensors
Georeferencing was performed using 1:50,000 scale topographic maps. A subset was taken to match the area before and after eruption. The study area is 1460 column and 1460 lines (73 km square).

The use of vegetation index was thought to be the most appropriate method to evaluate the extent of the damage in the Pinatubo area. Vegetation cover ratio was calculated by a kind of vegetation index (KVI)2) as equation (1). The equation for the KVI was derived as the difference between Near Infra-red (NIR) and Red (R) Band with a multiplication constant acting on Red Band as shown in formula (2).

VCR = DVI/ KVI max (1)
KVI = NIR- a x R - b (2)

These a and b are constant, a is obtained from the slope, and b in interception of Y axis of the soil line that constituted the description of bare soil on the plots of Red Band versus Near Infra-red Band.

An average value in the 4 pixels is assigned as the single value in the output image. After this treatment, one pixel size is 100 m by 100 m.

Determination of the degree of damage was derived from the difference of vegetation indices between the before- and after- eruption. The percentage of damage appears to show more reasonable behavior because we easily understand the situation of the damage. The percentage of damage (VCR (differ)) was calculated by formula (3).

VCR (differ) =

VCR(before)-VCR (after) VCR (before)

x 100

In further analysis, the areas of clouds, shadows, smokes and low vegetation before eruption were determined. This was accomplished by selecting training samples of sea, land, clouds, cloud shadows, and smokes, and subsequent maximum likelihood classifications of the subsets to extract the areas of cloud, cloud shadow, and smoke coverage. The low vegetated area before eruption was extracted using KVI value. By these processes, the information necessary for making was provided analysis area. The masking area had no data about the vegetation damage, so these data was estimated from surrounding data.

A histogram was used to demarcate the severity of damage into five percentage-based categories in order to facilitate human visual interpretation. This result was categorized according to the degree of damage on the vegetation into five classes such as 100-90%, 90-60%, 60-30%, 30-10%, and 10-0%.

The result of 3 weeks after eruption, 100-90% damage to the vegetation occupies the largest area. After 4 month, the least damaged vegetation area (0-10% damage) is dominated. Along the rivers and the areas near the top of the volcano remaining in the highly damaged (100-90% damage). Estimated damaged areas are listed on Table 5. The area of most severely damaged (90-100% vegetation degraded) had attained as much as 164,000 ha in the date of three weeks after the eruption. While the most severely damaged area was decreased to about 22,000 ha at the time of 4 months after the eruption, but more than 30% damaged area was still existed more than 190,000 ha. The situation of 9 months and 2 years after eruption is almost same to the condition after 4 months. After 4 years, vegetation was recovered a little.

Vegetation cover decrease ratio %	After 20 days 1991/07/05 1000ha(%)	4 months 1991/10/23 1000ha(%)	9 months 1992/03/16 1000ha(%)	2 years 1993/04/02 1000ha(%)	4 years 1995/07/14 1996/01/07 1000ha(%)
100-90	164(33)	22(4)	41(8)	57(11)	32(6)
90-60	123(25)	59(12)	35(7)	48(10)	34(7)
60-30	86(17)	101(21)	91(18)	93(19)	67(14)
30-10	38(8)	93(19)	104(21)	80(16)	113(24)
10-0	88(17)	218(44)	228(46)	221(44)	232(49)

Table 5 Vegetation cover decrease ratio by the eruption of Mt. Pinatubo