Planning a sound mission to acquire aerial photographs is critical to completing a vegetation mapping project successfully. The planned mission must ensure adequate image overlap, side-lap, radiometric balance, scale, and clarity. Stereoscopic viewing of aerial photos is only possible with overlapping photographs. Images must be clear throughout, with no "hotspots" (i.e., very bright areas typically near the center of the frame). Finally, the photos must be at the correct image scale. These parameters must be determined in the photo mission planning stage. Good aerial mapping firms use software to develop flight plans. The contractor must be able to explain how each parameter was calculated in planning the mission. Their numbers should roughly correspond to preliminary estimates completed by installation personnel. The number of frames required to cover the area of interest greatly affects the cost of acquisition.
The calculations used to estimate the total number of frames to complete a photo mission require several parameters (Avery and Berlin 1992). These parameters include:
· Dimensions of the Area of Interest (feet) — Estimate the rectangular dimensions of the area to be covered by the aerial photography. Typically, the area to be mapped is not square or rectangular. Therefore, use a box large enough to cover the entire area.
· Average Elevation of the Area of Interest (feet above mean sea level) — Use hard copy topographic maps (e.g., U.S. Geological Survey 7.5-minute topographic quadrangles) or, if available, a digital elevation model to estimate the average elevation of the area of interest.
· Desired Photographic Scale (representative fraction) — Based on the vegetation mapping objectives, select an appropriate photographic scale. The desired, or nominal, scale of vertical aerial photography is similar to the scale associated with maps. A small-scale photograph (e.g., 1:60,000) is acquired from a relatively high altitude, covers a larger area, and will supply less vegetation detail than a large-scale photograph (e.g., 1:12,000). Fewer small-scale photos will be needed to cover an area than with larger-scale photography.
· Desired Film Format (e.g., 9 in. by 9 in.) — As discussed earlier, the standard 9- by 9-in. format is probably the most appropriate for stereoscopic viewing. Other less common film formats include 9- by 18-in. and 7- by 7-in. frames.
· Camera Specifications — The primary variable of interest associated with the mapping camera is the focal length. Typical focal lengths range from 3 ½ to 12 in. A 6-in. focal length is common for large- and medium-scale photography, while a 12-in. focal length is used with high-altitude, small-scale images.
· Percent Overlap (%)— Assume a 60-percent overlap between consecutive photographs along the flight line. This amount of overlap ensures complete stereo coverage of the area of interest.
· Percent Side-lap (%)— Assume a 30-percent overlap between adjacent flight lines. This also helps to ensure that all of the area of interest is captured during photo acquisition.
Once all of these parameters have been determined, use the following flow of five equations to complete a preliminary photo mission plan:
(1) Estimate Flight Altitude: This number is calculated based on the desired photographic scale.
H = [(RFd)(f)] + (elevation)
where: H = flying height of the aircraft above the terrain (feet)
RFd = denominator of the representative fraction of the desired scale
f = focal length of the camera (feet)
elevation = average elevation of area of interest (feet)
(2) Calculate Ground Distance: This calculation estimates the lateral ground distance (along flight path) covered by each frame.
D = (RFd)(d)
where: D = lateral ground distance covered by a single photographic frame (feet)
RFd = denominator of the representative fraction of the desired scale
d = dimension of one side of a photo (based on the desired film format) (feet)
(3) Calculate Number of Flight Lines: The total number of flight lines needed to cover the area of interest uses the ground distance covered by each photo and the desired side-lap. Always round up to the nearest whole number (e.g., 6.3 becomes flight lines).
NL = {W/[(D)(Sg)]} + 2
where: NL = estimated number of flight lines
W = width of the area of interest (feet)
D = lateral ground distance covered by each frame (feet)
Sg = side-lap gained by each successive flight line = (100% minus desired side-lap %)
2 is to add an extra parallel flight line to each side of the area of interest; this ensures total coverage of the study site
Flight lines are typically oriented due north-south or due east-west. This assists the pilot during photo acquisition. Try to select the orientation of the flight lines so that they are parallel to the long axis of the area of interest (i.e., the long axis of the rectangle). This minimizes flight time by minimizing the number of turns during photo acquisition.
(4) Calculate Number of Photographs per Flight Line: This number uses the length of a single flight line to estimate the number of frames needed to provide stereo coverage of that line.
NP = {L/[(D)(Og)]} + 4
where: NP = number of photos to cover a single line
L = length of a single flight line (feet)
D = lateral ground distance covered by each frame (feet)
Og = overlap gained by each successive frame = (100% minus desired overlap%)
4 is a total of two photos added to each end of the flight line to ensure complete coverage of the area of interest
(5) Calculate Total Number of Photographs:
TP = (NP)(NL)
where: TP = total number of photographs to cover the area of interest
NP = number of photos to cover a single flight line
NL = number of flight lines to cover the area of interest
By developing a rough estimate of the total number of photos to acquire, the overall scope and objectives of a proposed project can be evaluated to determine whether aerial imagery can provide a cost-effective source of information.