intervals, between −3 and +3, for a total of 19 different EVs To mimic airborne imagery collection from a terrestrial captured (see Table 1). WB variable images were captured location, we attempted to approximate an equivalent path utilizing the camera presets listed in Table 1 at the EVs of 0, radiance to match a range of altitudes (750 to 1500 m) above −1 and +1, for a total of 27 images. Variable light metering ground level (AGL), that we commonly use as platform alti- tests were conducted using the “center weighted” light meter tudes for our simulated damage assessment research. Optical setting, at the EVs listed in Table 1, for a total of seven images. depth increases horizontally along with the zenith angle, from The variable ISO tests were carried out at the EVs of −1, 0, and a factor of one at zenith angle 0°, to around 40° at zenith angle +1, using the ISO values listed in Table 1, for a total of 18 im- 90° (Allen, 1973), and vertically, with a zenith angle of 0°, the ages. The variable aperture tests were conducted using 19 dif- magnitude of an object at the top of the atmosphere decreases ferent apertures ranging from f/2 to f/16, utilizing a different by 0.28 at sea level, 0.24 at 500 m, and 0.21 at 1,000 m above camera system from all the other tests, with ISO = 50, f = 105 sea level, to 0 at around 100 km (Green, 1992). With 25 mm, four different degrees of onboard vignetting control, and percent of atmospheric scattering due to atmosphere located EVs of −⅓, ⅔, 0, and +⅔, for a total of 228 images. between sea level and 1,000 m, this would mean a horizontal Two DSLR camera systems were used in this study, a Nikon image at 1,000 m would have 25 percent less path radiance D800E full frame camera paired with a Nikon DC-Nikkor 105 than the same image captured at sea level. Given both the mm f/2 to f/16 relative aperture lens, which was used for the non-linear and temporally varying nature of optical depth, a variable aperture study, and a Sony Alpha 65 APS-C camera 25 percent reduction in path radiance was an estimate used (Bockaert, 2006) paired with a Sony f = 18-55 mm f/3.5 to 5.6 for this study. We simulated vertical imaging altitudes of 750 relative aperture lens, which was used for the variable EV, WB, m and 1,500 m AGL, such that the 565 m to 1,125 m in the light meter, and ISO studies. The Sony a65, shown setup for horizontal plane, above ground level. our experimental imaging in Figure 3, would have been the In addition to images datasets generated for the change only camera utilized for this study, however the widest aper- detection noise minimization tests (described above), an- ture usable to capture the 55 mm images, f/5.6, is an aperture other set was created that simulated infrastructure damage. at which vignetting effects are negligible. For this reason, Simulated cracks were used to test the sensitivity of different the Nikon D800E with a wider f/2 aperture lens with more capture settings in detecting cracks in a wall or road as prox- pronounced vignetting effects was utilized for the aperture ies for infrastructure damage. Image pairs taken of simulated testing (DXOMARK, 2012). cracks simulated were analyzed to determine the settings for which the smallest crack can be resolved from difference Table 1. Experimental camera exposure variables and their images and to examine the influence ofEV settings on crack settings. detection. To assess the ability to detect damage signals, simu- EVs White Light Metering Relative lated cracks made out of black painter’s tape were placed on (19) Balance (9) EVs (7) ISO (6) aperture (19) a concrete wall with widths approximately equal to one, two, three, and four pixels (GSD = 4.5 cm). Vertical cracks were cre- -3 AWB -3 100 f/2 ated with varying widths, equal to one, two, three, and four -2 ⅔ 2500K -2 200 f/2.2 times the estimated image ground sampling distance (GSD), -2 ⅓ 3500K -1 400 f/2.5 and diagonal cracks were created equal to one, two, and four -2 4500K 0 800 f/2.8 times the estimated GSD, using matte black tape. An oblique -1 5500K +1 1600 f/3.2 imagery collection was performed with the Sony camera mounted on a tripod from the top of building 42 m vertically, 0 6500K +2 3200 f/3.5 960 m horizontally distant from the concrete wall. A second +1 7500K +3 f/9 collection (repetition of EV settings) made after the tape had +2 8500K f/10 been removed. Image pairs for each EV setting were regis- +2 ⅓ 9500K f/11 tered and then subset to the extent covering the simulated cracks and their surrounding background (concrete wall). The +2 ⅔ f/13 images for this test were collected with only a single WB of +3 f/14 AWB, compared to the nine WB settings captured for the noise f/16 minimization test imagery, all other settings utilizing the con- trolled constants from the noise minimization image set. The The Nikon D800E was used to test across-image variability ideal EV level is that which enables detection of the smallest during a single collection, utilizing a reflectance calibration simulated cracks, with the greatest temporal brightness differ- panel as a surface of uniform brightness. The reflectance ence across the simulated crack features. panel was positioned perpendicular to the sun, with the camera positioned as close to perpendicular with the sun and reflectance panel as possible, without having the shadow of Data Analysis the camera in the resulting image. Images were captured in Automated change detection products are typically based on the mid-afternoon, to lessen the chances of accidental overex- image differencing of one or more wavebands or transform im- posure when capturing images with a high f-stop. ages for a RSI pair (in our case), creating an output a raster with The Sony camera captured four image sets for two different band values representing the magnitude and sign of DN value scenes in support of dynamic range and spatial acuity tests changes. The most appropriate (if not optimal) imagery collec- with variable EV, WB, light metering, and ISO. Two collections tion settings were those combination of settings which produce were carried out in the afternoon with a solar elevation of images with the lowest root mean square difference (RMSD) val- 45°, and two collections were carried out in the late afternoon ues for DNs in the difference images, for test images or subsets with a solar elevation of 30°, to achieve variations in illumi- where scenes contained few actual land surface changes. nation conditions and degrees of shadowing. Images were Prior to image analyses, test images were downloaded, captured on clear and hazy days, and yielding image data sets sorted, and assessed for quality, for both the noise minimiza- influenced by two differing atmospheric optical conditions. tion and simulated crack signal maximization assessments. The sites chosen were selected to capture differing urban Test image pairs used for dynamic range and light metering, scene compositions, and to achieve an image viewing azimuth vignetting, and simulated crack signal assessments, required angle parallel to the solar azimuth at time of collection.

152 March 2018 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING