ES 771 Lecture
James S. Aber

Table of Contents
Ground observations Ground radiometer
Bidirectional reflectance Modeling BRDF
Kite aerial photography Related sites

Ground Observations

Ground observations are necessary in order to properly interpret remotely sensed imagery. The collection of ground data is often called ground truth, as a means to verify the identify of features. The collection of ground observations takes two general paths.

Ground-Based Radiometer

An example of ground-based radiometers is the FieldSpec® by Analytical Spectral Devices, Inc. in Boulder, Colorado. This portable instrument is designed to produce laboratory-quality spectral signatures in the field. In various configurations, it can detect ultraviolet, visible, and infrared energy (350 to 2500 nm wavelength). It can record 512 spectral channels with resolution of 3 nm and accuracy of ± 1 nm per channel. Solar radiation, reflected from ground objects, is collected via a fiber-optic cable that sends the "light" into the radiometer. Output from the radiometer is processed, displayed, stored, and analyzed by a notebook computer. Data are collected under field conditions that approximate routine remote sensing. Current cost of a complete FieldSpec setup is about $60,000. The geospatial analysis program at ESU has a smaller, hand-held unit (cost about $10,000).

FieldSpec® portable radiometer operated at the Tallgrass Prairie National Preserve, Chase County, Kansas by Duane Nellis (left) and his assistant, Yongyi Tao, who holds the sensor "head" over prairie grass.
Closeup view of the FieldSpec® portable radiometer. Sensor input is connected to the radiometer via a fiber-optic cable. Output of the radiometer is processed for display by the notebook computer. Photos © J.S. Aber.

Field spectrometry

Bidirectional Reflectance Distribution Function (BRDF)

The phenomenon of bidirectional reflectance, also known as multi-view angle (MVA) reflectance, is the variation in reflectivity depending on the location of the sensor in relation to the ground target and sun position (Asner et al. 1998). In conventional remote sensing, the camera or scanner looks straight down (or nearly so) to capture a vertical (nadir) view of the Earth's surface. However, some newer sensors are designed to acquire data in oblique views. The bidirectional reflectance distribution function (BRDF) refers to variations of reflectivity with different viewing angles, particularly within the solar plane. The solar plane is the vertical plane that contains the sun, ground target, and sensor. The following illustration demonstrates a typical BRDF acquired with the ASAS instrument.

Illustration of ASAS operation and typical BDRF.
Taken from (Ranson et al. 1994).

Note the position of the sun and differences in reflectivity both toward and away from the sun. Reflection back toward the sun is called backscatter, and reflections opposite the sun are forward scatter. The typical BRDF displays maximum reflectivity at the antisolar point, often called the hot spot or opposition effect, which is the position where the sensor is in direct alignment between the sun and the ground target. The cause of the hot spot is apparently the hiding of shadows at this position (Hapke et al. 1996). The absence of visible shadows causes the spot to appear substantially brighter than other views in which shadows appear. At the position opposite the antisolar point, another special lighting effect may happen. Sun glint is a mirror-like specular reflection often seen from water bodies.

The bright spot at scene center is an example of the hot spot or opposition effect at the antisolar point. At this position, no shadows are visible so the spot appears brighter than the surroundings. The opposition effect is most common over homogeneous forest or prairie vegetation. Ross Natural History Reservation, east-central Kansas.
The bright reflection from the water surface is an example of sun glint—a direct specular reflection like a mirror. Lake Kahola, east-central Kansas. Kite aerial photographs © J.S. Aber.

Reflectivity of vegetation is strongly anisotropic because of complex geometry of plant bodies and leaves in typical forest or prairie settings. The factors that influence BRDF for vegetation can be divided into landscape (macrostructural) and canopy (microstructural) features.

Some landscape and canopy structures that influence BRDF. Based on Schaaf and Strahler (1994) and Anser et al. (1998).
Landscape Structures Canopy Structures
Distribution vegetation types Leaf properties (specularity)
Canopy dimensions Leaf (and stem) area index (LAI)
Crown spacing Leaf and stem angle distribution
Shadowing patterns Multiple scattering interactions

Modeling Bidirectional Reflectance

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 others words ground truth. NASA has developed and operates two instruments for MVA remote sensing.

NASA's Parabola field radiometer for measurement of bidirectional reflectance (also known as omnidirectional or multidirectional reflectance). Three-channel (visible, near-infrared & mid-infrared) radiometer with two-axis scanning; designed for operation at low height from a vehicle-mounted boom, stationary tripod, or tower.

Parabola is capable of panning and tilting to acquire measurements in all look directions over a target area. However, it cannot view the same spot on the ground from different directions, so care must be taken in selecting a ground study site that is relatively homogeneous in its land cover and vegetation canopy.

Left: Closeup view of Parabola radiometer mounted on a boom. Taken from NASA.

Diagram of the viewing directions for Parabola.
Taken from NASA.

NASA's ASAS—Advanced Solid-state Array Spectroradiometer. A hyperspectral, multi-angle, airborne radiometer. Off-nadir tilting capability used for measurements of bidirectional reflectance of crops, forest, prairie, snow and soil. Designed to operate at 5000-6000 m (16,000 to 20,000 feet) altitude on a NASA C-130 or P3 aircraft (large multi-engine airplanes). Tilts +70° to -55° in direction of flight with cross-track field of view 19°.

The unique tilting ability allows the instrument to look at the same ground area from multiple view angles. Sixty-two channels in visible and near-infrared spectrum. Operated by NASA Goddard Space Flight Center since 1987.

Left: Example of ASAS image set from Prince Albert National Park, Saskatchewan, Canada. The scene includes a narrow lake within a boreal (spruce) forest. Note changing brightness levels associated with different viewing angles in the solar plane; antisolar point at -45°. Taken from Boreas 94 project summary (NASA).

Kite Aerial Photography

Kite aerial photography provides a low-cost means to acquire ground truth from heights of 50-150 m above the surface. Various types of film, digital and video cameras may be employed to capture images in all orientations relative to the sun and ground target. This MVA capability makes kite aerial photography a convenient means to depict BRDF for different types of land cover. The following examples illustrate variations in reflectivity for a spruce forest on the island of Vormsi, northwestern Estonia, 59°N latitude.

Mid-day view toward the northeast with sun behind the camera—note shadow of navigation tower, lower left. The scene is illuminated well with good color definition and few shadows within the forest canopy. All photos © J.S. Aber.
Mid-day view toward the east with sunshine from the right. The forest appears somewhat darker than above, because more shadows are visible. Note the lighthouse to far right; it appears in the next image.
Mid-day view toward the southeast looking toward the sun, which is positioned just right of view. Forest is quite dark, and sky is very bright. Sun glint reflects from the water surface to right. Lighthouse is visible at far left.

The following sequence of photographs depicts an agricultural landscape near Seroczyn in east-central Poland. Agricultural fields display two patterns—narrow strips of individual farmers and large rectangular fields of cooperative farms. Small forest patches occupy steeper slopes or stony soils. The pictures were acquired in late summer, after some crops had been harvested.

Mid-day view toward the southeast; note shadows cast by trees. Cloud shadows appear in the background. Kite flyers are visible at bottom of scene, just left of kite line. Overall the scene is relatively dark. All photos © J.S. Aber.
Mid-day view toward the northeast with sun behind the camera; note tree shadows. Scene is illuminated well with good color distinctions. Photo date 9/2000; © J.S. Aber.
Mid-day view with sun directly behind the camera. A marked hot spot is visible at the antisolar point, just below scene center. Note shadows of poles for power line across the scene. Photo date 9/2000; © J.S. Aber.
Mid-day view toward the southwest with sun in front of the camera; note tree shadows. Heavy shading renders the ground quite dark with poor depiction of colors, whereas the sky is quite bright. Photo date 9/2000; © J.S. Aber.

Related Sites


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