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Analysis Districts (AD) can be a individual or
multiple scenes footprints that have to be observed on the same date, and
analyzed as one. An AD can be
comprised of one or more scenes. An AD can be defined by either a scene edge
or a county boundary. Multi-temporal
AD's are possible as long as both dates in all scenes are the same.
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A single or multi-scene AD will use all
potential training fields for clustering/classification/estimation. Even
though a segment can lie in more than one scene, each segment can be
identified as belonging to only one AD.
Therefore, a segment can be used many times during the
clustering/classification stage, depending on the amount of scene overlap,
but each segment can be used only once for estimation.
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Preliminary definition of ADs usually only
considers the dates of imagery and the existence of cloud cover problems. A
slight re-definition might later be based on amount of training data
available or parameters required in the regression estimation process.
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Usually all parts of an AD are contiguous, but
holes might exist due to cloud cover problems.
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Having two dates per scene location is
preferred, one early in the growing season of the most important crops and
one later. For example, in the Mississippi River Delta Region, one scene from
April through May, and another from June through August would be selected;
these cover the spring planted crops such as rice, cotton, soybeans, corn and
sorghum.
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Two scene dates from the same sensor and same
scene location can be overlaid; in addition, Landsat 5 TM and Landsat 7 ETM+
can be overlaid with each other.
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If only one date can be found for a given scene
location, it would be selected based on the mid-growing season of the most
important crops. Anytime the vegetation of interest is active and not in
senescence, may work.
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In a two date image situation, some information
is usually gained from scenes that are too early, such as the distinction
between woods/pasture and bare soil.
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In a one date situation, scenes that are too
late tend to create confusion due to bare soil, harvested crops, or
non-vigorous vegetation. Obviously, scenes that are too early have bare soil
for crops.
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