longitudinal (x-coordinate) , the position of the dyed water mass must be 

 recorded from the time of injection to time t n , where n signifies the 

 limit of control area delineated by the grid. Photo coverage in time 

 increments is a first-order necessity for this purpose. The flight lines 

 for an area of 1,280.16 meters square (4,200 feet) can vary from a single 

 pass at 2,316.48 meters (7,600 feet) to multiple passes at lower altitudes. 

 Given a cluster of 70-millimeter cameras with 100-millimeter focal lengths, 

 the typical field of view for 1,066.8 meters (3,500 feet) (flight alti- 

 tude) is 591.62 meters square (1,941 feet) (a photo scale of 1:10,672), 

 typically with 30-percent sidelap and 50-percent endlap (Fig. 36). Expe- 

 rience indicates that 2 to 2.5 hours flight time is required for an area 

 of this size to complete advection of the 600 grams of dye used per in- 

 jection point, resulting in about 80 to 90 frames exposed per camera. 



b. Multispectral Photography . Aerial films should be exposed for 

 water, rather than land. The product can be considerably enhanced by 

 selecting the films in combination with filters responding to the band- 

 width at which either the fluorometric or colorimetric reflectance of the 

 dyes in the field is at maximum. Several film-filter combinations can be 

 selected in this manner, preferably one for each tracer and without spec- 

 tral overlap. The resulting system becomes multispectral (whether black 

 and white or color films are selected for each bandwidth) and the extended 

 dynamic range thus allows certain processing techniques to be applied and 

 analytical methods (color re cont ruction; density slicing) to be practiced 

 for the evaluation of the image content. 



If vertical photos are difficult to obtain, oblique photos can be used 

 by rectifying the images with the aid of an instrument such as the Zoom 

 Transfer Scope (Bausch and Lomb) (Fig. 37) . The scope functions as a 

 camera luoida, i.e., the superposition of two images, two maps or an image 

 and a map through an optical system becomes possible. Rotation, stretch- 

 ing and zoom capabilities of the scope allow the user to match control 

 points, scales, and within limits rectify the obliquity in an image for 

 the vertical resolution or vice versa. This instrument is useful in the 

 collation of information from imagery of varying scales onto a common 

 engineering map. Use of oblique photos in dispersing dye and the reduced 

 information are shown in Figure 38. 



Data to be extracted from photos include the measure of Vi, the 

 advective property of the dye plume. Vi can be estimated from the propa- 

 gation of the leading edge of the plume with time; however, it is not 

 indicative of the true transport of mass. To obtain the correct estimate, 

 the center of the propagated mass must be determined and its displacement 

 traced with elapsed time; e.g., methods used include: (a) The sampling 

 of dye patch from boats or with automatic samplers at various geographic 

 points and in time (Kilpatrick and Cummings, 1972); (b) the measurement 

 of the tracer concentration with a aerial Fraunhofer camera or Fraunhofer 

 line discriminator (Hemphill and Stoertz, 1969); and (c) the analytical 

 processing of imagery which includes density slicing. 



Although the last method is strictly qualitative, it can be quantized 

 with controlled sampling of the water mass during the course of photo- 

 graphic flights. 



64 



