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Figure 18.5
Field spectroradiometry sampling conducted August 14-17, 2001, at 4 of 13 wetland sites for
comparison to the PROBE-1 reflectance spectra. The procedure involved recording (A) reference
spectra and (B) vegetation reflectance spectra during midday solar illumination. Vegetation spectra
were recorded from 1 m above the vegetation canopy.
Transects along the edges of target-species stands were recorded using a real-time differential
GPS for sampled target species (Figure 18.3). Each of the two nontarget stands of vegetation was
delineated with a minimum of four GPS points, evenly spaced around the perimeter. Five GPS
ground control points (GCPs) were collected at Pointe Mouillee, generally triangulating on the
sampled areas of the wetland (Figure 18.2). GPS location points were recorded along with multiple
digital photographs, as necessary, to provide multiple angle views of each sample location. The
edge polygons, GPS points, GCPs, field notes, and field-based images (camera) were used to provide
details about ground data for imagery georeferencing, classification, and accuracy assessments.
A quadrat sampling method was used within each target-species stand to sample herbaceous
plants, shrubs, trees, and other characteristics of the stand (Mueller-Dombois and Ellenberg, 1974;
Barbour, 1987). Depending on stand size, 12 to 20 (nested) 1.0-m
quadrats were evenly
spaced along intersecting transects (Figure 18.4). The approximate percentage of cover and taxo-
nomic identity of trees and shrubs within a 15-m radius were also recorded at each quadrat. Where
appropriate, the terminal quadrat was placed outside of the target-species stand perimeter to
characterize the immediately adjacent area. This placement convention improved the accurate
determination of vegetation patch edge locations. The location of SAM classification output was
accomplished partly by identifying a uniform corner of each quadrat with the real-time differential
GPS to provide a nominal spatial accuracy of 1 m. Field data were collected to characterize both
canopy and understory in targeted wetland plant communities (Table 18.2).
Reflectance spectra were measured in the field for each of the target species at four selected
wetland sites (Site A, Site B, Site F, and Site J; Figure 18.2) on August 14-17, 2001, using a field
spectroradiometer (Figure 18.5). Field spectra collected from 1 m above the top of the
and 3.0-m
2
2
Phragmites
canopy were compared to PROBE-1 to confirm target species spectra at Pointe Mouillee and were
archived in a wetland plant spectral library.
18.3.3
Accuracy Assessment of Vegetation Maps
A three-tiered approach was used to assess the accuracy of PROBE-1 vegetation maps. This
approach included unit area comparisons with (1) photointerpreted stereo panchromatic (1999)
aerial photography (1:15,840 scale), (2) GPS vector overlays and field transect data from 2001
(Congalton and Mead, 1983), and (3) field measurement data (2002).
Pointe Mouillee 2002 sampling locations were based on a stratified random sampling grid and
provided to a field sampling team as a list of latitude and longitude coordinates along with a site
orientation image, which included a digital grayscale image of the site with the listed coordinate
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