Tier I: Chesapeake Bay-wide Aerial Survey

What is the Chesapeake Bay-wide Aerial Survey?

The Chesapeake Bay-wide Aerial survey is an annual aerial image gathering and assessment effort to determine the distribution and density of SAV in the Bay and its tidal tributaries. The effort is led by the Virginia Institute of Marine Science (VIMS) and is supported by the Chesapeake Bay Program and additional state and federal partners. It is the first tier of the Chesapeake Bay SAV Monitoring effort.

Aerial image of the Rappahannock River made at an altitude of 13,200 feet. — Dense SAV in the Rappahannock River is seen near the U.S. 301 Bridge in Port Royal, Virginia. The color aerial image was made at an altitude of 13,200 feet. ((Photo courtesy for Virginia Institute of Marine Science))

How does it work?

Conducted each year by VIMS, the survey involves flying an airplane outfitted with a multispectral digital mapping camera and an inertial measurement unit (IMU) over designated flight lines to obtain imagery. Flight lines are positioned in areas known to be inhabited by SAV and in areas where SAV may be expected to occur. Flights are prioritized for areas of significant SAV coverage and are timed to occur during the peak biomass of the SAV species known to inhabit the area. Guidelines for aerial imagery acquisition are strict; conditions (e.g., tidal stage, plant growth, sun angle, turbidity and wind) need to be nearly optimal for SAV detection.

Flight lines from the 2019 VIMS Aerial Survey. ((Map by Virginia Institute of Marine Science))

How is the imagery used?

Once obtained, the digital aerial imagery is visually examined to identify SAV beds. Images of areas with SAV present are georectified and orthographically corrected (a process that flattens out terrain and aligns the imagery to a coordinate system) to create a seamless image mosaic for the Bay. From these mosaics, outlines of SAV beds are hand-drawn and delineated using GIS software. SAV density within the beds is estimated into four classes: 1 (<10% coverage); 2 (10-40% coverage); 3 (40-70% coverage); or 4 (70-100% coverage), by comparing the image to a crown density scale (see Figure 2). Information from supplemental ground surveys, including the SAV Watchers program, is tabulated and entered into the GIS data. Area for all SAV beds is then calculated and reported.

Graphic shows the Crown Density Scale for SAV

SAV distribution data are organized according to the Chesapeake Bay Program’s Segmentation Scheme. 93 segments throughout the Bay grouped into four salinity zones: tidal fresh (<0.5 ppt); oligohaline (0.5-5 ppt), mesohaline (5-18 ppt), and polyhaline (18-25 ppt). These data are used to produce annual reports, tables, charts, and an interactive map that are available on the VIMS website.

Aerial Survey Reports and Data

The following reports, charts and tables are on annual SAV density and coverage throughout the Chesapeake Bay.

Interactive SAV Map

Visualized Chesapeake Bay SAV distribution data can be accessed through an interactive map hosted on the VIMS website. The map enables users to display many different SAV data and resources from 1971 to present day, including:

SAV species
Bed densities
Bed outlines
Flight lines
SAV Imagery
Long-term composite outlines
Comparison of years
Orthorectified imagery
Segment specific, historical SAV fact sheets

The visualization of this data allows resource managers to track, compare and analyze change in SAV distribution in specific locations over time throughout the Bay. It is a useful tool for SAV restoration site planning activities and for watershed organizations engaged in local-scale monitoring and a necessary tool for permitting and regulatory requirements.

The VIMS interactive SAV Map interface shows a zoomed-in area of the Middle River, Maryland, where SAV beds are clearly visible. ((Virginia Institute of Marine Science))

Bay-wide SAV Survey funders

The following state and federal agencies provide funding to the Bay-Wide Aerial SAV Survey.