Geoscience Reference
In-Depth Information
points were selected (
n
= 86). The supplied points represented the center point of mapped areas of
dense
and > 75% cover). Accordingly, the 86 sampling points selected
to support the validation and accuracy assessment effort contained no “false positive” control
locations. At each field validation sampling location, both 1-m
Phragmites
(> 25 stems/m
2
quadrats were used. Five
differentially corrected GPS ground control points were collected to verify the spatial accuracy of
field validation locations.
and 3-m
2
2
18.4 RESULTS
18.4.1
Field Reference Data Measurements
stand sampled at Pointe Mouillee was bounded on the eastern
edge by an unpaved road with two small patches of dogwood and willow in the north and a single
small patch of willow in the south (Figure 18.4). A mixture of purple loostrife (
The northernmost
Phragmites
Lythrum salicaria
)
and
stand was dry and varied
across the stand from clayey-sand to sandy-clay, to a mixture of gravel and sandy-clay near the
road. Litter cover was a constant 100% across the sampled stand; nontarget plants in the understory
included smartweed (
Typha
bounded the eastern edge of the stand. Soil in the
Phragmites
Polygonum
spp.), jewel weed (
Impatiens
spp.), mint (
Mentha
spp.), Canada
thistle (
Cirsium arvense
), and an unidentifiable grass. Cattail was the sole additional plant species
in the
canopy.
The southernmost Pointe Mouillee
Phragmites
stand was completely bounded by manicured
grass or herbaceous vegetation, with dry and clayey soil throughout. Litter cover was 100% and
nontarget plants in the understory included smartweed, mint, purple loosestrife, and an unidentifi-
able grass. Nontarget plants were not observed in the canopy. Comparisons of the two field-sampled
stands indicated that quadrat-10 region of the northernmost stand was the most homogeneous of
all sampled quadrats. Accordingly, field transect data were used to determine which pixel(s) in the
PROBE-1
Phragmites
data had the greatest percentage of cover of nonflowering
Phragmites
and the greatest
stem density (Figure 18.6).
18.4.2
Distinguishing between
Phragmites
and
Typha
are often interspersed within the same wetland, making it difficult to
distinguish between the two species. Because plant assemblage uniformity was measured in the field
(Figure 18.6), we could compare the PROBE-1 reflectance spectra of
Phragmites
and
Typha
Phragmites
within a single stand
of
(Figure 18.7). There was substantial spectral variability
among pixels within the northernmost stand of
Phragmites
(Plate 18.1) and with
Typha
Phragmites
(Plate 18.1). The greatest variability for
Phragmites
corresponded to the spectral range associated with plant pigments (470 to 850 nm) and
structure (740 to 840 nm). Comparison of reflectance characteristics in the most homogeneous and
dense regions of
Phragmites
(quadrat-10)
and
Typha
(quadrat-8) (Figure 18.4) indicated that
Phrag-
mites
was reflecting substantially less energy than
Typha
in the near-infrared (NIR) wavelengths and
reflecting substantially more energy than
Typha
in the visible wavelengths (Figure 18.7).
18.4.3
Semiautomated
Phragmites
Mapping
Based on the analyses of field measurement data, digital still photographs, digital video images,
field sketches, and field notes, we selected nine relatively pure pixels of
Phragmites
centered on
quadrat-10 in the northernmost stand (Figure 18.4). A supervised SAM classification of the PROBE-
1 imagery, using precision-located field characteristics, resulted in a vegetation map indicating the
