Monitoring Methods for SAV

SAV Species

For the purposes of this report, the term "submerged aquatic vegetation" (SAV) encompasses 23 taxa from 12 vascular macrophyte families, and 3 taxa from 1 freshwater macrophytic algal family, the Characeae. Our use of "SAV" excludes all other algae—both benthic and planktonic—that occur in Chesapeake Bay, its tributaries, and the Delmarva Peninsula coastal bays. Although these other algae species constitute a portion of the SAV biomass in this region, this survey does not attempt to identify, delineate, or discuss the algal component of the vegetation nor its relative importance in the flora. The aerial survey cannot differentiate epiphytic algae on submersed vascular plants or differentiate many benthic marine algae species, including many macrophytes, which can co-occur in the same SAV beds. To learn more about the 17 species of submerged aquatic vegetation are commonly found in Chesapeake Bay and its tributaries, visit our SAV species guide.

Aerial Photography

Aerial photography, in the form of multispectral digital imagery, is currently obtained by Midwest Aerial Photography (Galloway, OH) under contract with Air Photographics (Martinsburg, West Virginia). The multispectral imagery is acquired using a ZI DMC-II 230 multispectral (RGB,NIR) digital mapping camera and IMU with a 92-mm focal length, a 5.6-μm pixel size, and a 15552 x 14144 image size. This imagery is acquired at an approximate altitude of 13,200 feet, yielding a ground sample distance (GSD) of approximately 24 cm.

Flight lines—which differ in number from year to year based on weather and logistical factors—are designed to include land features necessary to establish control points for accurate mapping, which augment and check the IMU data. The flight lines used to obtain the photography are positioned to include all areas known to have SAV, as well as most areas that could potentially have SAV (i.e., all areas where water depths are less than 2 meters at mean low water). Flight lines are prioritized by sections and flights are timed during the peak growing season of species known to inhabit each area. In addition, priority is given to specific areas with significant SAV coverage.

Guidelines for acquisition of aerial imagery address tidal stage, plant growth, sun angle, atmospheric transparency, turbidity, wind, sensor operation, and land features. Adherence to these guidelines assures acquisition of imagery under nearly optimal conditions for detection of SAV, thus ensuring accurate photo interpretation. Deviation from any of these guidelines requires prior approval by VIMS staff. Quality assurance and calibration procedures are consistently followed. The altimeter is calibrated annually by the Federal Aviation Administration and the aerial camera is calibrated by USGS.

Camera settings are selected by automatic exposure control. To minimize image degradation due to sun glint, lines and frames are designed with 60% line overlap and 20% sidelap. The scale, altitude, camera settings, and focal-length combination are coordinated so that SAV patches of 1 square meter can be resolved. Ground-level wind speed is monitored hourly. Under normal operating conditions, flights are usually conducted under wind speeds less than 10 mph. Above this speed, wind-generated waves stir bottom sediments, which can easily obscure SAV beds in less than 1 hour. The pilot uses experiential knowledge to determine the acceptable level of turbidity that would allow complete delineation of SAV beds. During optimum flight conditions the pilot is able to distinguish bottom features such as SAV or algae at low tide. Excessively turbid conditions preclude photography.

Determination of maximum cloud cover level is based on pilot experience. Records of this parameter are kept in a flight notebook. Every attempt is made to acquire imagery when there is no cloud cover below 13,000 feet (cloud cover < 5% of the area covered by the camera frame). A thin haze layer above 13,000 feet is generally acceptable. Experience with the Chesapeake Bay has shown that optimal atmospheric conditions generally occur 2 to 3 days following passage of a cold front, when winds have shifted from north-northwest to south and have moderated to less than 10 mph. Within the guidelines for prioritizing and executing the photography, the flights were planned to coincide with these atmospheric conditions when possible.

Air Photographics coordinated the processing of all imagery, which is then delivered to VIMS on a portable hard drive. The imagery is then uploaded to VIMS image server and backed up on external hard drives in the case of unexpected drive failure.

Mapping Process

Multispectral aerial imagery is the principal source of information used to assess distribution and abundance of SAV in Chesapeake Bay, its tributaries, and the Delmarva Peninsula coastal bays. The digital aerial imagery is carefully examined to identify all photos with visible SAV beds, then these photos covering SAV beds are orthorectified and mosaicked to create a seamless orthophoto mosaic. Outlines of SAV beds are then interpreted on-screen, providing a digital database for analysis of bed areas and locations. Information from ground surveys is tabulated and entered into the SAV geographic information system (GIS).

USGS 7.5 minute quadrangle maps are used to organize the mapping process, including interpretation of SAV beds from aerial photography, mapping ground survey data, and compiling SAV bed area measurements. The SAV quadrangle index map gives locations of the 259 quadrangles in the study area that include regions with potential for SAV growth. Most quadrangles are sequentially numbered north to south for efficient access to data.

Orthorectification and Mosaic Production

Digital multispectral imagery is georectified and orthographically corrected to produce a seamless series of aerial mosaics following the standard operating procedures (SOP). ERDAS IMAGINE Photogrammetry image processing software is used to orthographically correct the individual flight lines using a bundle-block solution. Control points from Virginia Base Mapping Program (VBMP), Maryland Geographic Information Office, USDA National Agriculture Imagery Program (NAIP), and ESRI World imagery provide the exterior control, which is enhanced by a large number of image-matching tie points produced automatically by the software. The exterior and interior models are combined with a 10-meter resolution digital elevation model (DEM) from the USGS National Elevation Dataset (NED) to produce an orthophoto for each aerial photograph.

The orthophotographs for each flight line are mosaicked into an ArcGIS mosaic dataset, then each flight line is mosaicked together to create a single mosaic dataset for the entire Bay. This mosaic dataset is then shared as an ArcGIS image service.

Photo Interpretation and Bed Delineation

SAV beds are interpreted on-screen from the mosaicked image service using ESRI ArcMap GIS software. The identification and delineation of all SAV beds by photo interpretation utilizes all available information including knowledge of aquatic grass signatures on screen, distribution of SAV from prior years, current ground survey information, and aerial site surveys.

In addition to delineating SAV bed boundaries, SAV density is estimated within each bed by visually comparing it to an enlarged crown density scale similar to those developed for estimating crown cover of forest trees from aerial photography (Paine, 1981). Bed density is categorized into four classes based on a subjective comparison with the density scale. These are 1 (very sparse, <10% coverage); 2 (sparse, 10-40%); 3 (moderate, 40-70%); or 4 (dense, 70-100%). Either the entire bed or subsections of a bed are assigned a density number (1 to 4) corresponding to these density classes. Some beds are subsectioned to delineate variations of SAV density. Additionally, each distinct SAV bed or bed subsection is assigned an identifying one- or two-letter designation unique to its map. Coupled with the appropriate SAV quadrangle number and year of photography, these letter designations uniquely identify each SAV bed in the database.

Standard operating procedures (SOPs) have been developed to facilitate orderly and efficient processing of SAV maps and SAV computer files produced from them, and to comply with the need for consistency, quality assurance, and quality control. SOPs include a detailed procedure for orthorectification, mosaicking, and photo-interpretation; tracking sheets which record the processing of flight lines and quadrangles; and weekly summary progress reports of all operations.

Calculation of SAV Area

An ArcGIS geodatabase in a Universal Transverse Mercator (UTM) Zone 18 projection is used to calculate area in square meters for all SAV beds. These areas are summarized in tables by USGS 7.5 minute quadrangle, Chesapeake Bay Program and Delmarva Peninsula coastal bay segments, zone, and by state. Segment and zone totals are calculated using an overlay operation of segment and zone regions on the SAV beds.

Organizational Procedures for Analysis and Discussion

SAV distribution data are presented and discussed based on the 2003 revised Chesapeake Bay Program (CBP) segmentation and zonation scheme (DAWG, 1997). This segmentation scheme is mapped and listed by salinity regime.

The CBP Segmentation scheme defines 93 segments that are grouped into four salinity zones to reflect the communities of SAV species found in Chesapeake Bay: Tidal Fresh (less than 0.5 ppt), Oligohaline (0.5-5 ppt), Mesohaline (5-18 ppt), and Polyhaline (18-25 ppt).

SAV distribution in the Delmarva Peninsula coastal bays is presented and discussed separately from Chesapeake Bay. A fourth zone, the Delmarva Peninsula Coastal Bays Zone, is defined to include the region from Assawoman Bay to Magothy Bay and is subdivided into 10 segments: Assawoman, Isle of Wight, Sinepuxent

Ground Surveys

Ground surveys are accomplished by cooperative efforts from a number of agencies and individuals. Although not all areas of Chesapeake Bay are ground surveyed, the data provides valuable supplemental information. The ground surveys confirm the existence of some SAV beds mapped from the aerial imagery, as well as SAV beds that were too small to be visible on the imagery. The surveys also provide species data for many of the SAV beds. Ground survey information supplied to VIMS researchers is included on the SAV distribution and abundance digital maps and included in the VIMS SAV GIS Database. All ground survey data supplied to VIMS are tabulated in the ground survey table.