Study on the changing characteristics of land use as commons in Roviana, western province, Solomon Islands using high resolution satellite data and aerial photographs
Materials and Methods
In the field, ground control points and other ground truth data were acquired, using GPS systems of
Trimble Ltd.; TSC1 was used for collecting rover data and 4600LS for base data. In addition,
socio-economical, behavioral, demographic and other data were collected. Data used were (1)
IKONOS panchromatic and multispectral images acquired in July 2002, (2) digital colored aerial
photographs acquired in October 1991 (scale 1:20,000), and (3) a black-and-white aerial photograph in
May 1969 and in October 1947 (scale 1:50,000). Data (1) and (2) were used for analysis of change in
land use in Dekurana between 1991 and 2002. Data (1) and (3) were used for the land use in Dora
Island in 1947, 1969 and 2002.
The IKONOS panchromatic image was corrected based on ground control points after the differential
correction. Multispectral IKONOS image and aerial photographs were registered with this
georeferenced panchromatic image.
Analyses for Dekurana Data
IKONOS Image Analysis
Ten classes for land cover were made initially: (1) natural matured forest, (2) degraded/secondary forest,
(3) coconut grove, (4) garden, (5) tree plantation, (6) grassland, (7) bare soil, (8) house, (9) water, and
(10) cloud. A supervised maximum likelihood classification was performed for the whole image of
IKONOS multispectral data . In this stage, gardens and tree plantations could not be distinguished
properly from grasslands or bare soils. These two classes were, therefore, classified manually, making
AOI (area of interest) files on panchromatic image. Garden area contained small coconut groves and
other cash crop groves standing in the cleared lands for horticulture of tuberous crop. The areas with
planted trees which were big enough to be detected on the image (one year or more after planting) were
interpreted as tree plantation. These AOI files for gardens and tree plantations were then combined to
the computed image to produce the classification map in 2002 .
Aerial Photo Interpretation
Mosaic image in 1991 was created by combining five photographs. This was interpreted manually by
creating AOI files for each land class, taking same category in the IKONOS image analysis. Cloud
cover, which was little , was located in the area of natural forest. Therefore , this portion was classified
as natural matured forest. For the purpose of comparing the results of aerial photograp hs with IKONOS
image, cloud cover in IKONOS image was also applied to the corresponding area on the classified
image of aerial photographs .
Analyses for Dora Island Data
Gardens in Dora Island were interpreted manually on IKONOS panchromatic image and aerial
photographs in 1947 and 1969; photographs in 1991 did not cover this area.
Results and Discussion
New Georgia Island in 2002
Figure 2 shows the land cover of Dekurana area in 2002 based on the IKONOS image. The logging
roads, which were classified as bare soil, ran from south -east to north-west, and down to settlements.
Abandoned roads, which were classified as grassland, expanded into the inland forest. Gardens and
tree plantations were located along coast, river and logging road. Degraded forests by selective
logging could not be classified from natural forest because nine years had passed since the operation.
Figure 2. Land cover of Dekurana area in New Georgia Island in 2002 based on IKONOS multispectral
image.
Change of Land Cover of Dekurana Area between 1991 and 2002
Figure 3 shows the land cover of Dekurana area in 1991, based on the aerial photographs; cloud cover
corresponded to thick-cloud cover on IKONOS image in 2002. Deforestation occurred only along the
coast. Gardens located along river, coast (just behind mangroves), and surrounding areas of
settlements. Degraded/secondary forest, which also surrounded the settlements, included the area for
mixture of cultivated useful trees and natural vegetation, and the forest cut by means of the villager’s
portable saw-milling. In comparison with the land cover in Figure 2, this land cover was characterized
by expansion of tree plantations in large areas along coast and river, instead of the gardens.
Table 1 shows the change in areas from 1991 to 2002. Forest, which included natural and degraded
forest and coconut and useful plant groves, decreased from 1017.1 ha to 870.7 ha. The increase of
grassland and bare soil reflected the increase of anthropogenic disturbance on forest. The
comparability between the supervised classification computed from four-band high resolution image and
the manual classification of aerial photograph should be improved in further works.
Figure 3. Land cover of Dekurana area in New Georgia Island in 1991 based on aerial photographs.
