Geoscience Reference
In-Depth Information
Table 6.1
Land-Cover Classification Scheme and Definitions
Land Cover
Definition
Primary forest
Mature forest with at least 20 years growth
Secondary forest
Secondary succession at any height and less than 20 years growth
Transition
Area recently cleared, burned, or unburned and not currently in use
Pasture
Area planted with grass, ranging from overgrazed to bushy
Crops
Area with agriculture, including perennial and annual crops
Bare soil
Area with no vegetation or low, sparse vegetation
Water
Waterbody, including major rivers, water streams, and reservoirs
term residents and who were knowledgeable about historical LU practices in the region. Nearly
half of the members in each of the nine groups were farmers who settled prior to 1986.
An introductory meeting was conducted with each group to provide a hard copy (false-color
composite) of the 1999 ETM
image with parcel boundaries overlaid and to solicit comments and
observations regarding farm locations, significance of color tones on the image, and clarification
of LU practices and associated cover types. All participants were then asked to indicate retrospective
and current LU for their parcels and for other parcels with which they were familiar. Any questions
that could not be answered by individuals were referred to the group for discussion, elaboration,
and decision making. For each identified cover type, we annotated and labeled polygons on stable
acetate overlaid on the false-color composite image. Each polygon consisted of a homogeneous
area labeled as one of seven LC types for each year corresponding to the dates of the Landsat
images used in the study.
Notes were taken during the interview process to indicate the date each farmer started using
the land, areas of the identified LC types for each of the 3 years considered in the study (1986,
1994, 1999), changes over time, level of uncertainty expressed by participants while providing
information for each annotated polygon, and other information farmers considered relevant. After
each meeting, the research team traveled the main roads in the area just mapped by the farmer
association and compared the identified polygons with what could be observed. The differences
between the cover type provided by the farmers and what was observed were minimal. In areas
where such meetings could not be organized, the research team traveled the feeder roads and
annotated the contemporary LC types that could be confidently identified.
Field data were collected for over 1500 polygons, including all seven LC types of interest. We
considered this to be an adequate sample for image classification and validation of our maps.
Although an effort was made to ensure all land cover types were well represented in the database,
some types such as bare soil were represented by a relatively small sample sizes (
+
n
< 200 pixels).
6.2.3
Data Processing
More than 1000 polygons identified during the farmer association interviews were screen-
digitized and field notes about the polygons were compiled into a table of attributes. Independent
random samples of polygons for each of the seven land-cover types were selected for use in image
classifier training and land-cover map validation. Although the number of homogenous polygons
annotated in the field was large, polygons varied greatly in size from
1000 ha and were
not evenly distributed among the seven cover types (Table 6.2). For cover types that had a large
number of polygons, half of the polygons were used for classifier training and half for map
validation. For two cover types, however, the polygon samples were so large in area (and therefore
contained so many pixels) that they could not be used effectively because of software limitations.
The primary forest and pasture cover type polygons were therefore randomly subdivided so that
only one half of the pixels were set aside for both classifier training and for map validation (i.e.,
one quarter of the total eligible data pixels were used for each part of the analysis). However, this
approach did not yield a sufficient number of sample polygons for some of the more rare cover
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