Leica Geosystems

Case Study

Digital Imaging Provides Clarity to Florida Seagrass Monitoring

The Southwest Florida Water Management District protects the water supplies of 4.7 million people while also maximizing the environmental, economic and recreational benefits of water as a natural resource. Since the 1980s, the district has relied on seagrass monitoring as an indicator of overall ecosystem health in its five major estuaries. Working closely with Photo Science, SWFWMD has devised a process using digital aerial imagery for mapping seagrass that is accurate, efficient, and environmentally safe.

The Southwest Florida Water Management District protects the water supplies of 4.7 million people while also maximizing the environmental, economic and recreational benefits of water as a natural resource. Since the 1980s, the district has relied on seagrass monitoring as an indicator of overall ecosystem health in its five major estuaries. Working closely with Photo Science, SWFWMD has devised a process using digital aerial imagery for mapping seagrass that is accurate, efficient, and environmentally safe.

Florida is winning the water-quality battle in its estuaries and tributaries. Beginning in the 1950s, the state saw the health of these vital water bodies decline as unchecked agricultural, wastewater and other practices dumped excessive nutrients and pollution into the system and dredging operations tore up shallow marine vegetation, including seagrasses. These underwater plants contribute to, and depend upon, clean water in their ecosystems.

“Seagrasses are an indicator of water quality because they need light to photosynthesize, but sunlight only reaches them if the water is clean and clear,” said Kris Kaufman, South West Florida Water Management District (SWFWMD) Staff Environmental Scientist.

SWFWMD is responsible for protecting water resources in Tampa Bay, Charlotte Harbor, Sarasota Bay, Lemon Bay and St. Joseph Sound/Clearwater Harbor. The District has emerged as a sophisticated user of geospatial technologies to implement and monitor its management activities.

Looking for Clarity
SWFWMD first tracked historical seagrass gains and losses dating to the 1950s using black-and-white film photography from federal archives. The District initiated its own aerial mapping program in 1988 with acquisition of color aerial film photography on a two-year cycle. In 2006, the program made the transition from film to digital image acquisition and processing.

“Going digital was a turning point for our project because the resolution and clarity of the imagery provided better information in deeper water,” said Kaufman.

Two years earlier, Photo Science Inc., a geospatial solutions firm headquartered in Lexington, Kentucky had been awarded the seagrass mapping contract with SWFWMD and completed its first collect with a film camera. But shortly after, Photo Science purchased a Z/I Imaging DMC digital mapping camera, a large-format frame sensor designed for photogrammetric mapping.

“The most important benefit of the DMC that we saw for this type of project was the superior radiometric resolution,” said Gary Florence, Photo Science Vice President.

That translated into richer information content in the resulting imagery. The broader radiometric range gave the image analysts more detailed spectral information to work with in delineating and classifying underwater features, such as seagrass beds. Just as importantly, the one-foot spatial resolution of the DMC appealed to the district’s scientists because smaller features could be discerned deeper in the water and delineated more accurately.

Processing the Data
The crucial first step is flight planning with water clarity a key factor. Each estuary is treated as a single mission with the flight lines generated in advance by flight planning software. Once conditions are right, one of Photo Science’s twin-engine Aero Commanders departs for an altitude of 10,000 feet. In the air, the DMC is operated as it would be for most other projects.

“We have an airborne GPS/IMU [inertial measurement unit] that captures location and attitude data for each image frame,” said Florence. “Using the GPS/IMU over such a flat terrain, we don’t need to survey ground control points in advance.”

Once the aircraft lands, the hard drives are removed from the digital camera and sent to the Photo Science facility in Lexington, Kent., where the DMC Post Processing Software performs initial processing and color-balancing. Quality checks of thumbnail images confirm the scenes are suitable for seagrass mapping. The digital processing workflow continues with Intergraph GeoMedia and ImageStation OrthoPro software packages used to orthorectify the image frames.

“Color-balancing is a delicate process because we can manipulate the imagery to see far down into the water column, but it can’t be manipulated so much that land features become excessively washed out,” said Florence.

Classifying the Beds
Feature extraction and classification are conducted manually by photo interpreters in a heads-up digital environment using ArcGIS software. The Photo Science photointerpreters examine each image tile and delineate and classify underwater features, such as bare seafloor, oyster bars, shoals, tidal flats, and seagrass beds. Due to their importance, these beds are further classified according to their spatial distribution – patchy or continuous.

“The key is to determine whether seagrass is there, which can be tricky because its spectral signature can be similar to algae,” said Florence. “During the mapping process, our photo interpreters adjust the histogram distribution of colors to help further accentuate underwater features. ”

The one-foot spatial resolution of the imagery plays an important role in the success of the entire project because seagrass gain or loss in a specific area differs by only a few percentage points from one biennial inventory to the next. So the interpreters have to be exact in delineating the beds and making a precise classification of the spatial distribution according to SWFWMD specifications. To ensure this is accomplished, Photo Science uses a ground truthing method developed for this project.

“We attached an underwater video camera called a SeaViewer to a pool-cleaning pole and dip it into the water from a small boat,” said Florence. “The underwater videos show us what features exist in specific areas, and we use that information to verify and/or adjust our delineations and classification.”

Using the video ground truthing as their guide, the photo interpreters accurately classify the coverage of each seagrass bed. These classifications are represented as vectors in the GIS dataset. ArcGIS software is used to digitally compare the current year’s classification map against previous. Change detection vectors are categorized by colors to reflect an Increase, Decrease, or No Change in seagrass beds over time.

Photo Science delivers the classification maps with change detection vectors and the orthoimagery to the SWFWMD as end products. For all five estuaries, the mapping covers more than 2,400 square miles.

Mapping Success
The seagrass mapping project has been a success on several levels. Most importantly, it has revealed a steady growth in seagrass beds for the past decade, indicating water quality in all of the Southwest Florida estuaries has improved significantly. In terms of the monitoring project, SWFWMD says the switch to digital imaging was a turning point due to the speed and accuracy of the mapping.

Corporate Profile

Leica Geosystems and Z/I Imaging – Airborne Sensor Solutions from Hexagon Geosystems
Hexagon Geosystems’ newly formed Geospatial Solutions Division is bringing together Airborne Sensor solutions from Leica Geosystems and Z/I Imaging.   With over 80 years experience, Leica Geosystems is a global leader in the design, delivery, and support of airborne digital and lidar sensors for the geospatial marketplace. Along with the well-known Leica RC30, Leica Geosystems’ airborne sensor portfolio today includes a wide range of innovative technologies and products such as the Leica RCD30 series of medium-format digital frame cameras, the Leica ADS pushbroom sensors, the Leica ALS Lidar series, and the Leica IPAS GNSS/IMU solutions. For nearly the same 80 years, Zeiss and later Z/I Imaging have set standards in the airborne photogrammetric market with such products as the RMK series and the DMC. Today, Z/I Imaging continues to drive innovation and productivity with the new DMC II range of digital mapping cameras. Sensors from Leica Geosystems and Z/I Imaging are fully integrated in a suite of end-to-end workflow solutions. These include flight planning, GNSS/IMU processing, as well as the most comprehensive post-processing tools for the delivery of map products and 3D models in the fastest time possible. Hexagon Geosystems’ combined airborne sensor portfolio of Leica Geosystems and Z/I sensors offers complete solutions for almost all airborne mapping applications, plus all business models, and continues to give each sensor owner the highest and most consistent return on investment. In addition to world-class sensors, Hexagon Geosystems offers a full range of product support, service, and training with offices located in over 25 countries worldwide.  
 

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