The 2015 aerial photography was obtained primarily by Air Photographics (Martinsburg,
West Virginia) from multispectral digital imagery . The multispectral imagery was 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. That imagery was acquired at an approximate altitude of 13,200 feet, yielding a ground sample distance (GSD) of approximately 24 cm. This imagery was supplemented for a small set of areas with black and white aerial photographs 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. The black and white 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 RC-30 camera to acquire IMU data.
The 186 flight lines, which cover 3,937 flight line kilometers, were flown. These lines were desinged to include land features necessary to establish control
points for accurate mapping, augementing and checking the IMU data. 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 imagery address tidal
stage, plant growth, sun angle, atmospheric transparency, turbidity,
wind, sensor operation, and land features. Adherence to the guidelines
assured acquisition of imagery 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. To
minimize image degradation due to sun glint, lines and frames were designed 60% line
overlap and 20% sidelap. The scale, altitude, camera settings, 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 maximun cloud cover
level was based on pilot experience. Records of this parameter were
kept in a flight notebook. Every attempt was made to acquire
imagery when there was no cloud cover below 13,000 feet. Cloud
cover did not exceed 5% of the area covered by the camera frame. A thin
haze layer above 13,000 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 imagery. Digital imagery was delevered on a portable hard drive. Black and wihote film was scanned photogramettrically and also delivered on a hard drive/Each
black and whte 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.