AERIAL PHOTOGRAPHY

The 2003 aerial photography was primarily obtained by Air Photographics (Martinsburg, West Virginia). Additional photography for the lower Potomac River was obtained from the Metro Washington Council of Governments and from St. Mary's County, Maryland.

The 2003 aerial photography obtained by Air Photographics was acquired using a Wild RC-20 camera, with a 153 mm (6 inch) focal length Aviogon lens and Agfa Pan 200 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.

The 193 flight lines, which cover 4,318 flight line kilometers, were numbered and included land features necessary to establish control points for accurate mapping (See map, Figure 7). 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 in the Middle and Upper Zones (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 to occur during the peak growing season of species known to occur in each section. In addition, specific areas with significant SAV coverage were given priority. Dates of photography are noted on each SAV quadrangle map (Appendix B.)

Guidelines for acquisition of aerial photography (Table 1) 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 where 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.

Due to adverse weather conditions and Hurricane Isabel, Air Photographics was not able to acquire aerial photography for a large portion of the lower Potomac River, the western shore of the Bay between the Potomac and Rappahannock rivers, and the Eastern Shore from Eastern Neck Island to Swan Point. Additional aerial photography for the lower Potomac River with sufficient water clarity for mapping SAV was obtained from the Metro Washington Council of Governments and Saint Mary's County.

The color aerial photography obtained from the Metro Washington Council of Governments was flown on September 17, 2003 for the Department of Environmental Programs. A total of 65 frames at a scale of 1:12,000 from seven flight lines were scanned to map SAV for the lower Potomac River.

The color aerial photography obtained from Saint Mary's County was flown on March 27, 2003 and covered the entire county at a one foot resolution. The entire dataset was acquired as a single Mr. SID compressed image file.