19. REMOTE SENSING OF ALBEDO USING THE BRDF IN
RELATION TO LAND SURFACE PROPERTIES
181
features of the underlying landscape, indicating that the retrievals and
subsequent considerable extrapolation in solar zenith angle do not lead to
noisy or otherwise ill-determined results. For example, changes in the
number of observations in different areas, due to differences in cloud cover,
are not evident in the albedo. The bright areas seen are those of cities. For
example, Boston may be identified on the central east coast, Providence at
the end of an inlet west of Cape Cod, New York City and Long Island are on
the lower left. A web of bright urban built-up areas along the major
highways is visible. The relatively built-up Connecticut river valley in the
lower center, the Hudson river valley running north-south along the left side
of the image, and the Mohawk river valley with its agricultural lands running
east-west on the western left edge of the image show bright surfaces as well.
Dark areas also correspond to surface features: the dark region in the upper
left corner is the Adirondacks area, which is heavily forested, and one can
see the Berkshires with their forests running north-south between the
Connecticut and the Hudson rivers. The woody Catskills are to the lower
left. Generally, New England is densely vegetated, in many parts with dense
forest. Small details, like the dark spur near the northern end of the
Connecticut river valley, as well as bright spots representing small towns
throughout the image correspond reliably to ground features as identified, for
example, on Landsat imagery (Schaaf et al., 1998).
The physical properties of the landscape viewed are more reliably
indicated by the images shown in panels C and D of Figure 1. The first
shows the Normalized Difference Vegetation Index (NDVI), computed from
the atmospherically corrected reflectances and maximum value composited.
Some residual gaps remain in this image as the NDVI can be computed only
for pixels which had at least one observation from the AVHRR. The image
confirms the densely vegetated state of the region and the effects of human
impact in urbanized areas, along highways, and in some agricultural areas
associated with the river valleys. Panel D is a red-NIR composite of New
England acquired by the AVHRR on January 15, 1996 after a snowstorm
had blanketed the region. Trees in the region are mostly barren, although
coniferous species grow in some areas or are part of a mixed forest. The
snow has fallen from the trees to the ground, as is evident from the
obscuration of the bright snow background in the vegetated areas. This
image is a map of large-scale openness of the landscape, indicating where
the background is freely visible and where it is hidden by trees. The image
demonstrates that indeed the Connecticut river valley is more fractured in
terms of canopy closure than the surrounding forests, that there are open
areas along the Hudson river valley and in the Mohawk valley area. The city
RELATION TO LAND SURFACE PROPERTIES
181
features of the underlying landscape, indicating that the retrievals and
subsequent considerable extrapolation in solar zenith angle do not lead to
noisy or otherwise ill-determined results. For example, changes in the
number of observations in different areas, due to differences in cloud cover,
are not evident in the albedo. The bright areas seen are those of cities. For
example, Boston may be identified on the central east coast, Providence at
the end of an inlet west of Cape Cod, New York City and Long Island are on
the lower left. A web of bright urban built-up areas along the major
highways is visible. The relatively built-up Connecticut river valley in the
lower center, the Hudson river valley running north-south along the left side
of the image, and the Mohawk river valley with its agricultural lands running
east-west on the western left edge of the image show bright surfaces as well.
Dark areas also correspond to surface features: the dark region in the upper
left corner is the Adirondacks area, which is heavily forested, and one can
see the Berkshires with their forests running north-south between the
Connecticut and the Hudson rivers. The woody Catskills are to the lower
left. Generally, New England is densely vegetated, in many parts with dense
forest. Small details, like the dark spur near the northern end of the
Connecticut river valley, as well as bright spots representing small towns
throughout the image correspond reliably to ground features as identified, for
example, on Landsat imagery (Schaaf et al., 1998).
The physical properties of the landscape viewed are more reliably
indicated by the images shown in panels C and D of Figure 1. The first
shows the Normalized Difference Vegetation Index (NDVI), computed from
the atmospherically corrected reflectances and maximum value composited.
Some residual gaps remain in this image as the NDVI can be computed only
for pixels which had at least one observation from the AVHRR. The image
confirms the densely vegetated state of the region and the effects of human
impact in urbanized areas, along highways, and in some agricultural areas
associated with the river valleys. Panel D is a red-NIR composite of New
England acquired by the AVHRR on January 15, 1996 after a snowstorm
had blanketed the region. Trees in the region are mostly barren, although
coniferous species grow in some areas or are part of a mixed forest. The
snow has fallen from the trees to the ground, as is evident from the
obscuration of the bright snow background in the vegetated areas. This
image is a map of large-scale openness of the landscape, indicating where
the background is freely visible and where it is hidden by trees. The image
demonstrates that indeed the Connecticut river valley is more fractured in
terms of canopy closure than the surrounding forests, that there are open
areas along the Hudson river valley and in the Mohawk valley area. The city