United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/S4-91/014 Aug. 1991
EPA Project Summary
Wetlands Detection Methods
Investigation
K. H. Lee
The purpose of this investigation was
to research and document the applica-
tion of remote sensing technology to
wetland detection and mapping. Vari-
ous remote sensing sensors and plat-
forms are evaluated (1) for suitability
to monitor specific wetland systems;
(2) for their effectiveness in detailing
the extent of wetlands; (3) for their ca-
pability to monitor changes; and (4) for
the resulting relative cost-benefits of
implementing and updating wetlands
databases.
The environment to be monitored
consists of physiographic and ecologi-
cal wetland resources affected directly
or Indirectly by anthropogenic activity.
Aircraft and satellite remote sensing
can be used to record and assess the
condition of these resources. Monitor-
ing of environmental conditions is
based on the observation and interpre-
tation of certain landscape features.
Although some forms of monitoring are
continuous, resource monitoring from
aircraft and satellite platforms Is peri-
odic in nature, with change being docu-
mented through a series of observa-
tions over a given span of time.
This report summarizes the findings
of a bibliographic search on the meth-
ods used to inventory and/or detect
changes in wetland environments. The
bibliography contains numerous cita-
tions and is not intended to be all-
inclusive. Books, major journals, and
symposium proceedings were exam-
ined. The findings documented will
provide the potential user with a basic
understanding of remote sensing tech-
nology as it is applied to wetland moni-
toring and trend analysis.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The purpose of this report is to present
an overview of wetlands mapping proce-
dures, emphasizing the role of satellite
and aircraft imagery and their interpreta-
tion; field verification; and compilation of
all these data. The procedures for map-
ping changes in the extent and types of
wetlands is generally based on a com-
parison of earlier (historical) remote sens-
ing imagery to recent imagery.
The detail, accuracy, and precision of
the mapping depends on what results are
desired from the final report; the availabil-
ity of the necessary remote sensing data;
the technical personnel and equipment on
hand; and the specific budget for the
project.
Wetlands are difficult to map because
of (1) water level fluctuations, (2) the many
differing types of wetland settings, (3) the
difficulties of ground travel in placing geo-
graphic control point markers in this soggy
terrain, and (4) the changes in the bound-
aries of wetlands types caused by varia-
tions in the hydrologic cycle with its sea-
sonal, annual, and long-term fluctuations.
Furthermore, other changes may have
been created by anthropogenic factors,
Printed on Recycled Paper
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United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/S4-91/014 Aug. 1991
EPA Project Summary
Wetlands Detection Methods
Investigation
K. H. Lee
The purpose of this investigation was
to research and document the applica-
tion of remote sensing technology to
wetland detection and mapping. Vari-
ous remote sensing sensors and plat-
forms are evaluated (1) for suitability
to monitor specific wetland systems;
(2) for their effectiveness in detailing
the extent of wetlands; (3) for their ca-
pability to monitor changes; and (4) for
the resulting relative cost-benefits of
implementing and updating wetlands
databases.
The environment to be monitored
consists of physiographic and ecologi-
cal wetland resources affected directly
or indirectly by anthropogenic activity.
Aircraft and satellite remote sensing
can be used to record and assess the
condition of these resources. Monitor-
Ing of environmental conditions is
based on the observation and interpre-
tation of certain landscape features.
Although some forms of monitoring are
continuous, resource monitoring from
aircraft and satellite platforms is peri-
odic in nature, with change being docu-
mented through a series of observa-
tions over a given span of time.
This report summarizes the findings
of a bibliographic search on the meth-
ods used to inventory and/or detect
changes in wetland environments. The
bibliography contains numerous cita-
tions and is not intended to be all-
inclusive. Books, major journals, and
symposium proceedings were exam-
ined. The findings documented will
provide the potential user with a basic
understanding of remote sensing tech-
nology as it Is applied to wetland moni-
toring and trend analysis.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The purpose of this report is to present
an overview of wetlands mapping proce-
dures, emphasizing the role of satellite
and aircraft imagery and their interpreta-
tion; field verification; and compilation of
all these data. The procedures for map-
ping changes in the extent and types of
wetlands is generally based on a com-
parison of earlier (historical) remote sens-
ing imagery to recent imagery.
The detail, accuracy, and precision of
the mapping depends on what results are
desired from the final report; the availabil-
ity of the necessary remote sensing data;
the technical personnel and equipment on
hand; and the specific budget for the
project.
Wetlands are difficult to map because
of (1 ) water level fluctuations, (2) the many
differing types of wetland settings, (3) the
difficulties of ground travel in placing geo-
graphic control point markers in this soggy
terrain, and (4) the changes in the bound-
aries of wetlands types caused by varia-
tions in the hydrologic cycle with its sea-
sonal, annual, and long-term fluctuations.
Furthermore, other changes may have
been created by anthropogenic factors,
Prated on Recycled Paper
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such as encroaching urban and agricul-
tural developments; by stream channel al-
terations; and drainage and damming con-
structions.
To avoid the problems associated with
seasonal changes in wetland boundaries,
both the remote sensing data and the
field verification data should be collected
at the same time of the year so that the
same boundaries will be in the same geo-
graphical positions.
Procedures
The major part of mapping the wetlands
is generally done through the interpreta-
tion of remote sensing imagery. This is
considered here to be the satellite imag-
ery, airborne radar imagery, airborne multi-
spectral digital imagery, vertical overlap-
ping aerial photographs (black and white,
color, and color infrared [CIR]), and to a
minor extent color photographs (in hand-
held cameras) and video tapes taken from
aircraft.
Satellite imagery has a small scale, and
generally cannot be viewed stereoscopi-
cally. This contrasts with aerial photo-
graphs which have a much larger scale,
ranging normally from about 1:10,000 to
1:58,000, and which can be viewed ste-
reoscopically.
The larger scale of the aerial photo-
graphs with its smaller mapping units of
wetlands cannot be correlated directly with
the larger units of the satellite imagery.
This problem can be largely overcome by
combining the smaller units (from the aerial
photographs) until they correspond to the
larger units of the satellite imagery. This
correspondence has been as high as 90
percent in some studies.
Several different types of satellite imag-
ery are available: the MSS (Multispectral
Scanner) has four spectral bands: visible
green, visible red, and two reflectance in-
frared bands having about an 80-meter
ground resolution.
The TM (Thematic Mapper) satellite has
six spectral bands with a 30-meter resolu-
tion, and a thermal infrared band with a
120-meter resolution. Both these satel-
lites are in polar orbits and can collect
spectral information every 16 days over
the same surface area.
TM is better suited for wetlands map-
ping than the MSS because of better spa-
tial resolution (30 m), seven narrower spec-
tral bands sensitive to differences in spec-
tral reflectance of vegetation, and a higher
level of quantization (i.e., digital numbers
for TM = 256, for MSS = 128).
SPOT satellite imagery (a French satel-
lite) has a 10-meter black and white reso-
lution, and a 20-meter resolution for three-
band multispectral images; this is a finer
resolution than the LANDSAT MSS and
TM images, but it has a fewer spectral
bands and a smaller areal coverage. This
satellite can be oriented during its orbit so
as to produce stereoscopic coverage; it
has a repeat cycle of 26 days. SPOT
data can be integrated with LANDSAT TM
data to provide a composite view, up-
graded by the finer spatial features of
SPOT and the finer spectral resolution of
TM.
One advantage of satellite imagery is
color enhancement of scenes to empha-
size certain features of the landscape, such
as different types or stages of vegetation,
etc; this procedure is useful in wetlands
interpretation. Conversely, the stereo-
scopic exaggerated relief (normally about
three or four times) observed in the stereo
images of the normal aerial photographs
can help to delineate the low-lying areas
of wetlands.
Many series of aerial photographs are
available for most areas in the States.
Typical scales normally range from around
1:10,000 to 1:58,000. An ongoing pro-
gram by the USGS will cover the entire
United States with small-scale CIR
1:58,000 aerial photography. The CIR
photography seems to be the preferred
type for most landscape characterization
(which includes wetlands mapping).
The above indicates the many options
in remote sensing imagery available, and
the planning necessary to select the most
advantageous imagery for primary baseline
mapping. To map the changes over time
in wetland areas, historical aerial photo-
graphs are generally used; these can ex-
tend back as much as 50 years. A series
of these older photographs can show the
development and trend of the present sta-
tus of the wetlands now being mapped.
Side-looking radar is little used for wet-
lands mapping but has been successful.
Video imagery has been little used, its
resolution less than aerial photography.
Further work on optimizing spectral bands
must be done before video can be used
as an additional data source.
Previous Mapping
Previous mapping of wetlands provides
information on any later changes in the
character of the wetlands. However, three
serious problems in integrating previous
wetlands data into current mapping are:
1) The classifications can be based on
different criteria so that the units
mapped previously are not the same
as those being mapped for the
present project.
2) The areal precision of ground con-
trol points predating Global Position-
ing Systems (GPS) is usually poor
for correlating any specific feature,
point, or contact with later mapping.
Compilation of such data is not fea-
sible in many cases.
3) The differences in scales between
the previous mapping and later map-
ping may make correlations difficult
if not impossible; this may also con-
trol the differences in the ground or
spectral criteria included in the vari-
ous wetlands classifications.
A classification system developed by
the USGS in 1976 covers all remotely-
sensed resource data, not just the wet-
lands. A wetlands classification system
(Cowardin System) was developed by the
Forest Service in 1979 to replace a 1955
system, and is largely used or modified.
Field Verification
Until recently, the problem of locating
field geographic control points exactly has
been a major problem. The seasonal or
annual changes in this water-dominated
environment, plus even changes by indi-
vidual storms and the general thick veg-
etation cover, together with the difficulties
of field personnel movement within the
wetlands, can prevent the establishment
of adequate recognizable control points.
The one outstanding technical develop-
ment which apparently overcomes much
of this problem is the development of GPS
developed by the US military.
At the present time, the satellite data
are available only during a certain few
hours in the day so that field work must
be coordinated to take advantage of the
specific schedules of the satellites to re-
ceive their transmissions for site control.
Geographic Information System
The Geographic Information System
(GIS) is distinguished from other data man-
agement systems by its ability to perform
spatial analyses with multiple levels of data
in a selected geographic area. The GIS
is a convenient and organized method for
analysis of wetlands and allows for a co-
hesive database into which additional in-
formation can be incorporated and refine-
ments made.
Computerized GISs are widely utilized
to store, query, retrieve, display, and man-
age large amounts of digital data as-
sembled from many sources. The ability
to geometrically transform and integrate
multiple data types is very important when
accounting for differences in scale, map
projections, spatial resolutions, and carto-
graphic coordinate systems.
Both the raster and vector systems are
used in the GIS. For example, geometric
or overlay (point) operations are easier to
•&U.S. GOVERNMENT PRINTING OFFICE: 1991 - 548-028/40062
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