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Application of New Technologies for Aquatic Plant Management
Introduction
Accurate maps of the aquatic plant distribution within a reservoir or riverine system
are essential for an effective aquatic plant control program. However, developing
such maps is not an easy task, and managers have problems obtaining good estimates
of total plant-infested area and identifying the number of acres affected by each
aquatic plant type.
Many operational and management decisions depend on these estimates, including decisions
that affect budget requests and contract specifications, selection of priority treatment
areas and control methods, and timing of chemical applications to achieve effective
control. A proven and cost-effective method for surveying, mapping, inventorying,
and analyzing aquatic plant distributions would be highly beneficial to project
managers.
Aquatic plant-infested areas are traditionally estimated by visual inspection, or
they are delineated and inventoried using aerial photographic-based methods. An
experienced resource manager can make fairly reliable estimates of aquatic plant
acreage in some areas of the project such as boat ramps or marinas, because the
size and distance between certain physical features are known. However, use of this
method for large open areas or complex shoreline areas has proven unreliable. Conducting
an inventory of the plant-infested area at a project using aerial photographic methods
is labor intensive, is generally too expensive to repeat every year, and most importantly,
can take months to complete.
Such success has recently been achieved in mapping aquatic plant distributions using
digital satellite images or other digital image data. The analysis of these digital
images generally requires specialized software and large hard drives for data storage.
Further, the acquisition and processing of the images usually takes a long time.
A method for field mapping and computer analysis of aquatic plant distributions,
based on Global Positioning System (GPS) and Geographic Information System (GIS)
technologies, has been evaluated under the U.S. Army Corps of Engineers' Aquatic
Plant Control Research Program in cooperation with the Lake Seminole Resource Management
Office, Mobile District.
Methods
GPS is a surveying technology that uses small, inexpensive handheld receivers to
capture satellite signals. GIS technology is based on the digital manipulation and
analysis (processing) of spatial data. When used together, these technologies provide
the capability to accurately survey, describe, analyze, and map aquatic plant distributions.
The procedure is quick simple, and is conducted under direct control of the project
manager.
GPS (Global Positioning System)
GPS is a surveying technology that uses small, inexpensive handheld receivers to
capture satellite signals. GIS technology is based on the digital manipulation and
analysis (processing) of spatial data. When used together, these technologies provide
the capability to accurately survey, describe, analyze, and map aquatic plant distributions.
The procedure is quick simple, and is conducted under direct control of the project
manager.
The term "Global Positioning System" refers to a system of 24 radionavigation satellites
maintained by the U.S. Department of Defense (DoD). Radionavigation is a method
of determining a geographic position (latitude and longitude) on the earth by measuring
the travel time of an electromagnetic wave between the transmitter and receiver.
The GPS satellites are the transmitters, and the receivers can be positioned anywhere
on the ground, in the sea, or in the air (Figure 1). The orbits of the satellites
are arranged to provide continuous (24-hour) global coverage.
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Figure 1
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The small size and low cost of commercially available GPS equipment enable project
managers and field personnel to acquire and use them in their daily routines. A
GPS receiver properly mounted and used on a boat can continuously record the geographic
location (latitude and longitude) of the boat while stationary or in motion. If
an airboat moves along the edge of an aquatic plant bed ( Figure 2), the location
of the edge of the plant bed is captured and stored by the onboard GPS receiver.
This basic location information is the beginning of an aquatic plant distribution
map.
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Figure 2
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Techniques for surveying aquatic plant distributions using a GPS receiver mounted
on an airboat have been designed and tested by WES in a cooperative effort with
the Lake Seminole Resource Management Office.
The basic field equipment needed for GPS-based surveying is shown in Figure 3. The
small domed antenna intercepts the satellite signals and transfers them to the receiver
for preliminary processing. Figure 4 shows two types of GPS antennas mounted on
the cage of an airboat. A six-channel receiver can track up to eight satellites.
The antenna and receiver are powered by an external battery pack or 12-volt DC battery
while in the field. For setup and testing in the office, a wall outlet or a 12-volt
DC power supple can be used.
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Figure 3
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Figure 4
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Preliminary data processing takes place in the receiver, and the data are stored
in the data recorder. The recorder includes a keypad and a small digital display,
which provide the human interface for the system, as well as some system software.
Different types of data recorders are available, with a range of data storage capacities.
Descriptive information related to plants or other features can be added to the
GPS data file as needed using barcode techniques. A barcode wand can be connected
to the data recorder for this.
At Lake Seminole, WES employed the barcoding option to add plant type identifiers
to the GPS data file. As the airboat navigates the edge of the plant bed, the plant
type as well as the location are recorded. The user decides the number of barcodes
needed (one for each plant type, open water, etc.) and what the verbal plant description
willl be (for example, "hydrilla" or "cut grass"). The barcodes themselves are generated
by part of the GPS software and printed on a laser printer. A small wand is used
to pick up the barcode symbol during the survey and store it with the GPS data (Figure
5). The barcode technique also works well when surveying stationary facilities such
as signs, docks, ramps, or camping pads. If the system is not set up for barcoding,
the identifiers can be accessed by a menu-driven process on the display screen of
the data recorder.
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Figure 5
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Data stored in the field recorder must be transferred to a computer for additional
processing. Each commercial vendor has its own software package that must be provided
with the GPS unit. The data transfer from the field recorder to the personal computer
is simple. A cable links the field recorder to a serial (communications) port of
the computer, and easy-to-use software takes care of the transfer. The processing
of the data into real-world (geographic) coordinates is a menu-guided procedure.
The result of data processing is a set of real-world (X,Y) coordinates tracking
the position of the receiver during the survey and information (from the barcode)
identifying or describing the important features at the survey points.
GIS (Geographic Information System)
Most GPS softwares allow the transfer of processed (real world) data to other software
packages such as a Geographic Information System (Figure 6). GIS is a technology
for processing and analyzing digital geographic spatial data. "Digital" means the
data are in machine-readable format. The term "processing and analyzing" encompasses
almost any set of computer functions. "Geographic" means that the data are referenced
to the earth. "Spatial" refers to the data model space. That is, the data are derived
from and represent the physical shape, size, form, and substance of something that
is real in the landscape, wateshed, or reservoir.
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Figure 6
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The GIS will accept a string of processed coordinates from the GPS software and
reformat the coordinates into a digital map of aquatic plant distribution. Descriptive
or quantitative information (plant type, density, etc.) is linked directly to this
digital aquatic plant map.
The digital map can be processed or analyzed in a variety of ways to support the
planning and execution of a control program. Project managers can use the GIS to
easily determine the total acerage of aquatic plant beds of each plant type, to
calculate the length of the plant-water interface, or to compute the distance of
a nuisance plant bed from critical habitat or public access points.
As the aquatic plant control operation is carried out, additional information can
be added to the GIS database to document the type of control procedures used. For
instance, the geographic limits of a herbicidal application would be entered into
the database and documented by the date of application, the chemical and application
rate, the contractor, and the total amount of chemical applied during the treatment.
One of the great benfits of building and maintaining the GIS database is the capability
to easily compare several years of aquatic plant distribution data. Over time, the
patterns of plant spread and decline can be mapped, analyzed, and directly compared
to the known limits of the herbicide applications. Using these analysis techniques,
the results or effectiveness of the herbicide treatment can be accurately documented
and stored in the database. This capability provides a new and powerful planning
tool for managers.
A database with only the boundaries of aquatic plant beds depicted would be of limited
use in designing an effective control program. In fact, the location and extent
of the plant beds is only one factor that must be considered in scheduling control
and selecting the type of control.
The distribution of the aquatic plant infestations is most meaningful when compared
against the other physical and cultural resources of the project (Figure 7). For
aquatic plant management applications, a GIS database should include, among other
things, the shoreline, public access points (boat ramps, campgrounds, and picnic
areas), small boat channels, locations of any threatened or endangered species,
and critical habitats.
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Figure 7
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Accuracy Factors
Several factors affect the horizontal accuracy achieved using GPS surveying techniques.
Among these factors are the use of a local base station, degradation of the satellite
signal by DoD, and local environmental conditions.
A base station is a second GPS receiver, set up over a fixed point of known location,
for collecting the same satellite signals as the mobile receiver. Figure 8 shows
the base station at Lake Seminole. The best location for a base station is a bench
mark. Bench marks are rarely convenient, so any point with good vertical location
that has been carefully surveyed can serve as a base station location. The satellite
signal record captured by the base station is used to improve the horizontal positioning
of mobile receivers. Many mobile receivers can be serviced by a single base station.
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Figure 8
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Mobile receivers operated without a base station typically achieve horizontal positional
accuracies of 12 to 40 meters, depending on local environmenta; conditions and satellite
signal degradation. The horizontal position of a mobile receiver operated simultaneously
with a base station can be corrected to 1-to 5-meter accuracies. The newest GPS
systems on the market offer options that can achieve sub-meter horizontal accuracies
under the right conditions.
A second factor affecting positional accuracy is the quality of the satellite signal
itself. The DoD can deliberately degrade the satellite signal, causing inaccurate
horizontal position calculations. This DoD practice is called "selective availabilty".
Fortunately, the use of a base station can almost completely negate the effect of
the most common signal degradation method.
Local environmental conditions can interfere with signal reception. Heavy tree canopy,
large buildings, or hills may obstruct the sky. Disturbance of the earth's atmosphere
and ionosphere may cause delays in the signal which translate into positional errors.
Summary
GPS and GIS technologies are revolutionizing surveying, mapping, monitoring, and
resource management activities worldwide. These two technologies, properly integrated,
provide low-cost surveying, mapping, and resource analysis tools. These tools have
great potential to support field personnel and managers as they strive to meet the
challenges of ecologically sound multiple-use management.
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