The 2014 aerial photography was obtained primarily by Air Photographics (Martinsburg,
West Virginia) using a
Wild RC-30 camera, with a 153 mm (6 inch) focal length Aviogon lens and Agfa Pan
80 film, mounted in the bottom fuselage of a Piper Aztec, a twin engine
reconnaissance aircraft. Photography was acquired from an altitude of approximately
12,000 feet, yielding 1:24,000 scale photographs. A Novatel DL dual frequency GPS and an Applanix Phalanx IMU was attached to the camera to acquire IMU data. Multispectral digital imagery was acquired on June 15, 2014 over the mid-bay islands and the Eastern Shore using a ZI DMC-II 230 multispectral (RGB,NIR) digital mapping camera with a 92 mm focal length, a 5.6 μm pixel size, and a 15552 x 14144 image size. That imagery was acquired at an approximate altitude of 16,300 feet, yielding a ground sample distance (GSD) of approximately 30 cm. Additional imagery for the Potomac River, which happened to be acquired under conditions that were suitable for delineating SAV, was obtained from the USDA Aerial Photography Field Office NAIP Program (http://www.apfo.usda.gov).
The 168 flight lines, which cover 3,693 flight line kilometers, were
numbered and included land features necessary to establish control
points for accurate mapping if IMU data was not available. The flight lines used to obtain the photography were 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 were less than two meters at mean low water).
Flight lines were prioritized by sections and flights were timed during the peak growing season of species known to inhabit each area. In addition, specific areas with significant SAV coverage were
Guidelines for acquisition of aerial photography address tidal
stage, plant growth, sun angle, atmospheric transparency, turbidity,
wind, sensor operation, and land features. Adherence to the guidelines
assured acquisition of photography under nearly optimal conditions for
detection of SAV, thus ensuring accurate photo interpretation.
Deviation from any of these guidelines required prior approval by VIMS
staff. Quality assurance and calibration procedures were consistently
followed. The altimeter was calibrated annually by the Federal Aviation
Administration and the aerial camera was calibrated by USGS.
Camera settings were selected by automatic exposure control. Sun
angle was measured with a sensor on the plane. Flight lines were
plotted on 1:250,000 scale maps to allow for overlap of photography. To
minimize image degradation due to sun glint, the camera was equipped
with a computer controlled intervalometer which established 60% line
overlap and 20% sidelap. An automatic bubble level held the camera to
within one-degree tilt. The scale, altitude, film, and focal length
combination was coordinated so that SAV patches of one square meter
could be resolved. Ground-level wind speed was monitored hourly. Under
normal operating conditions, flights were 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 one
hour. The pilot used experiential knowledge to determine the acceptable
level of turbidity that would allow complete delineation of SAV beds.
During optimum flight conditions the pilot was able to distinguish
bottom features such as SAV or algae at low tide. Excessively turbid
conditions precluded photography. Determination of optimum cloud cover
level was based on pilot experience. Records of this parameter were
kept in a flight notebook. Every attempt was made to acquire
photographs when there was no cloud cover below 12,000 feet. Cloud
cover did not exceed 5% of the area covered by the camera frame. A thin
haze layer above 12,000 feet was generally acceptable. Experience with
the Chesapeake Bay has shown that optimal atmospheric conditions
generally occur two to three 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 film. A 9-inch by 9-inch,
black-and-white contact print was produced for each exposed frame. Each
photograph was labeled with the date of acquisition as well as the
flight line number. Film and photographs are stored under appropriate
environmental conditions to prevent degradation.
Turbid conditions in 2014 prevented the acquisition of imagery suitable to map the Southern Virginia Coastal Bays.