Emergent
Aquatic Vegetation
Mapping Swan
Lake, Nicollet County
For
this study, we
used an IKONOS
image acquired
on September 1,
2001 of the Swan
Lake area. Due
to the size of
Swan Lake and
the abundance
of aquatic plants
in the lake,
the collection
of reference
data would be
very difficult
without
the aid of modern
technology. We
used Global
Positioning System
(GPS) technology
on a field pen computer
with software capable
of displaying the
satellite data.
While in the field,
we identified different
types of aquatic
vegetation and located
them directly on
the IKONOS imagery
using the field
computer.
Being
able to accurately
identify specific
locations on the
image while in
the field was
especially useful
on this large
wetland. Having
the image available
quickly after
its acquisition
for use in reference
data collection
was also a significant
advantage in field
sampling because
we could identify
unique areas with
different spectral-radiometric
responses on the
image and target
them for field
identification.
We targeted emergent
vegetation for
the evaluation,
but also noted
the location of
submerged vegetation
appearing at the
surface.
The
first step for
the Swan Lake
IKONOS image was
to separate wetland
features from
terrestrial features
by digitizing
the aquatic terrestrial
boundary around
the entire wetland
and all islands.
We identified
this boundary
using spectral-radiometric
differences and
spatial patterns
visible on the
image. We then
subset the image
with the wetland
polygon to mask
out all terrestrial
features and create
a wetland-only
image.
Click
on map to mask out
land surrounding
Swan Lake.
Click
again to return to non-mask image.
Swan
Lake has a maximum
depth of two meters,
clear water throughout,
and an abundance
of aquatic vegetation.
Consequently,
we assumed that
aquatic vegetation
was present throughout
the wetland. An
aquatic vegetation
survey conducted
by researchers
at Minnesota State
University, in
which the presence
or absence of
27 species of
aquatic plants
at 118 evenly
distributed sample
points on the
lake was recorded,
verified this
assumption.
The
next step was to
stratify the wetland
into emergent and
submergent vegetation
by performing an
unsupervised classification.
Using the field
reference data,
we identified five
different emergent
classes on the emergent
vegetation images
and recoded the
images to create
an emergent vegetation
map. We repeated
this procedure
for the thick submerged
vegetation image
and identified two
submerged classes.
Finally, we created
an Aquatic Plant
Classification Map by
overlaying the
submerged aquatic
plant map and
the emergent aquatic
plant map over
the panchromatic
image.
Global
Positioning
System (GPS) is
a "constellation" of
24 well-spaced
satellites
that orbit
the Earth
and make it
possible for
people with
ground receivers
to pinpoint
their geographic
location.
