Geoscience Reference
In-Depth Information
of wetland vegetation can be the inaccurate assessment of mixtures of biotic and abiotic wetland
characteristics, even when wetland vegetation is predominated by a single taxon, such as
Phragmites
(Figure 18.6). For example, those bands observed in the near-infrared wavelengths for
Phragmites
may have caused image classification confusion (Plate 18.1). Heterogeneity and interspersion of
different wetland species are also thought to contribute to a relatively wide range of reflectance
values observed within wetland stands. Although water was not present at the selected Pointe
Mouillee sample locations in 2001, changes in hydrology and variability in soil moisture could
also contribute to inaccurate wetland classification. Thus, the biological and physical characteristics
of wetland plant communities at the time of imagery collection must be factored into the analysis.
To improve the accuracy of PROBE-1
-
derived maps we accounted for plant community het-
erogeneity by: (1) selecting plant taxa that were least likely to exist in diverse, heterogeneous plant
communities; (2) using GPS points with a nominal spatial accuracy that exceeds that of the imagery
data for locating sampled quadrats, stand edges, and ground control points; (3) acquiring a variety
of remote sensing data types to provide a range of spectral and spatial characteristics; (4) collecting
relevant ecological field data most likely to explain the differences in spectral reflectance charac-
teristics among pixels; (5) using archived aerial photography to assess and understand site history;
and (6) collaborating with local wetland experts to better understand the ecological processes at
the site and the historical context of changes.
18.6 CONCLUSIONS
The use of hyperspectral data at Pointe Mouillee demonstrated the spectral differences between
Phragmites
and
Typha
. Spectral differences between taxa are likely attributable to differences in
chlorophyll content, plant physical structure, and water relations of the two taxa. The combined
use of detailed ecological field data, field spectrometry data, and multiscalar accuracy assessment
approaches were instrumental to our ability to validate mapping results for
Phragmites
and provide
important information to assess the future coastal mapping efforts in the LGL. Additional classi-
fication and accuracy assessment procedures are ongoing at 12 other wetland study sites to determine
the broader applicability of these techniques and results (Lopez and Edmonds, 2001; Figure 18.2).
Other important ongoing research related to advanced hyperspectral wetland remote sensing
includes: (1) improving techniques for separating noise from signal in hyperspectral data, (2)
determining the relevant relationships between imagery data and field data for other plant species
and assemblages, (3) calibrating sensor data with field spectral data, (4) merging cross-platform
data to improve detection of plant taxa; and (5) employing additional assessment techniques using
field reference data.
The results of this study describe the initial steps required to investigate the correlations between
local landscape disturbance and the presence of opportunistic plant species in coastal wetlands.
These results support general goals to develop techniques for mapping vegetation in ecosystem
types other than wetlands, such as upland herbaceous plant communities. The results of this and
other similar research may help to better quantify the cost-effectiveness of semiautomated vegetation
mapping and accuracy assessments so that local, state, federal, and tribal agencies in the LGL can
decide whether such techniques are useful for their monitoring programs.
18.7 SUMMARY
The accuracy of airborne hyperspectral PROBE-1 data was assessed for detecting dense patches
of
Phragmites australis
in LGL coastal wetlands. This chapter presents initial research results from
a wetland complex located at Pointe Mouillee, Michigan. This site is one of 13 coastal wetland
