Geoscience Reference
In-Depth Information
10.3.6 Post-Classification Image Processing
After correcting for errors, the classifications were converted from raster data types
to vector format by first converting the classification to a grid, then converting the
grid to a shapefile in ArcToolbox (ESRI, ArcGIS 8.3). The resulting shapefile
contained the spatial and attribute data, and most importantly the land cover values,
from the original raster classification. After additional processing was performed
to ensure standardized nomenclature between the two datasets, the agriculture
and nonagricultural classifications were merged into one vector classification cont-
aining six classes: agriculture, forest, impervious, grass, bare soil, water/wet vege-
tation. Agriculture, early agriculture, and green agriculture were merged into a
generalized “agriculture” class. The goal of merging classes was to flatten seasonal
trends and variation to give a generalized long term picture of land cover during the
study period. Similarly, water and wet vegetation were merged into one class due
to short-term and seasonal fluctuations in water levels caused by wind direction,
seiche and rainfall. Variability between water and wet vegetation, specifically in
estuarine environments, occurs at a finer temporal scale than that of land cover, so
distinguishing between these two land cover types based on a single satellite image
is not useful to this particular analysis.
10.4 Results
10.4.1 Land Cover
Agriculture is by far the dominant land cover type in the Tribs watersheds (about
67% of the 450 km drainage area) (Fig.
10.3
). Of the four watersheds, Crane Creek
has the highest proportion of agriculture (about 72%) followed by Turtle, Cedar and
Wolf/Berger (69, 64, and 60% respectively) (Table
10.3
). All of the watersheds
have agricultural land use above the 50% threshold that Wang et al. (
1997
) found
to affect stream biotic integrity. Grass represents the second most dominant (11%)
land cover type across all four watersheds. Cedar Creek has the highest percentage
of grass (14%) and Turtle had the lowest (7%). Grassy areas represent mainly
residential and urban land uses, although a few natural grasses may have been
included during image analysis. Forest and impervious surfaces each represent
about 8% of the Tribs land cover. The forested portion of each watershed is between
7.03 and 7.61% with the exception of Turtle Creek, which is 11% forested. Cedar
creek has the most impervious surfaces (10.83%) and Turtle the least (5.64%)
Water/wet vegetation represented just over 4% of the total study area. The majority
of water is found at the mouths of the streams and represents estuarine sample sites.
Bare ground only represents a minimal proportion (2%) of the land cover.
