Bidirectional Reflectance Distribution Function

Qualitative variations in scene brightness and contrast depending on viewing angle and sun position are commonplace in small-format aerial photography, as noted above. The goal of the bidirectional reflectance distribution function (BRDF) is to model these variations quantitatively (Lucht, 2004). The BRDF is based on viewing and illumination angles as well as complex geometrical and optical properties of objects in the target scene. Considerable effort has been made to understand BRDF better for various types of land cover, particularly different vegetation canopies. One approach is to model mathematically the reflective plants and canopy geometry. Some models are based on radiative properties of plants (Nilson and Kuusk, 1989), whereas others depend on geometric-optical considerations (Schaaf and Strahler, 1994).

In either approach, the models can be tested against actual sensor measurements, in other words ground truth. Such field experiments begin with ground-based goniometers (Fig 4-13). A radiometer moves around the target object in order to collect reflectivity values from all azimuthal and zenithal directions at close range (2 m). From slightly higher vantages, NASA's Parabola radiometer can be operated from a vehicle-mounted boom up to 5 m high or suspended from cables attached to towers up to 30 m high (Bruegge et al., 2004). Parabola is capable of panning and tilting to acquire measurements in all look directions over a target area. Airborne BRDF instruments include POLDER and ASAS, which are typically flown at heights of 3000 m or higher (Schaaf and Strahler, 1994; Leroy and Breon 1996).

Light source v Sled with radiometer

Azimuthal arc

FIGURE 4-13 Schematic diagram of the field goniometer system (FIGOS) for multiview-angle, closeup measurement of reflectivity for targets under natural conditions. Adapted from Bruegge et al. (2004, fig. 5-10).

Figos Goniometer Developed

FIGURE 4-13 Schematic diagram of the field goniometer system (FIGOS) for multiview-angle, closeup measurement of reflectivity for targets under natural conditions. Adapted from Bruegge et al. (2004, fig. 5-10).

Representative ASAS results of such field measurements are presented for a spruce forest in Maine (Fig. 4-14). Three general observations have come from field observation of multiview-angle reflectivity of vegetation canopies (Murtha et al., 1997).

• Canopy reflectance is greatest when the sun is behind the sensor, in other words the camera records light that is backscattered from the canopy toward the sun. Maximum reflectance is located at the antisolar point, which produces a hot spot in the image due to shadow hiding.

• Canopy reflectance increases overall with higher solar zenith angles.

• Canopy reflectance is relatively uniform for vertical or near-vertical images, particularly those acquired with narrow fields of view.

One goal of BRDF research is to relate ground and aerial multiview-angle effects to observations collected from space-based remote sensing systems. Between ground-based and relatively high airborne BRDF instruments, however, the height range from 30 m to 1000 m is

The Parabola Iii Brdf

FIGURE 4-14 Multi-directional reflectivity for spruce forest in red (A) and near-infrared (B). In each case, the peak in reflectivity represents the hot spot at the antisolar point. The bowl of low reflectivity values indicates increased shadowing looking toward the sun. Adapted from Schaaf and Strahler (1994, fig. 3).

a little-explored interval for multiview-angle reflectivity. Small-format aerial photography is particularly well suited within this range to acquire all viewing angles from vertical to high oblique. Thus, SFAP represents an effective and inexpensive means for ultralow-height field experiments and testing of BRDF models. This is an application for which small-format aerial photography may play an increasingly important role in the future.

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