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The MODIS LAI product algorithms were considerably more complex. The primary approach
for calculating LAI involved the inversion of surface reflectance in two to seven spectral bands and
comparison of the output to biome-specific look-up tables derived from three-dimensional canopy
radiative transfer modeling. All terrestrial LC was assigned to six global biomes, each with distinct
canopy architectural properties that drove photon transport equations. The six biomes included
grasses and cereal crops, shrubs, broadleaf crops, savannas, broadleaf forests, and needle forests.
The secondary technique was invoked when insufficient high-quality data were available for a given
compositing period (e.g., cloud cover, sensor system malfunction) and calculated LAI based on
empirical relationships with vegetation indices. However, a deficiency inherent with the second
approach was that NDVI saturates at high leaf biomass (LAI values between 5 and 6). The
computational approach used for each pixel was included with the metadata distributed with each
data set.
4.3 METHODS
Here we describe a field sampling design and data acquisition protocol implemented in 2002
for measuring
forest canopy properties for the analysis of correspondence to MODIS satellite
NDVI and LAI products. The study objective was to acquire field measurement data to evaluate
LAI and NDVI products using
in situ
measurement data and indirectly using higher-spatial-resolu-
tion imagery sensors including Landsat Enhanced Thematic Mapper Plus (ETM
in situ
) and IKONOS.
+
4.3.1
Sampling Frame Design
Six long-term forested research sites were established in the APB (Table 4.1). The objective
was to collect ground-reference data using optical techniques to validate seasonal MODIS NDVI
and LAI products. Baseline forest biometrics were also measured for each site. Five sites were
located in the Piedmont physiographic region and one site (Hertford) in the coastal plain. The
Hertford and South Hill sites were composed of homogeneous conifer forest (loblolly pine),
Fairystone mixed deciduous forest (oak/hickory), and Umstead mixed conifer and mixed forest,
and both Duke and Appomattox sites contained homogeneous stands of conifer and deciduous
forest managed under varying silvicultural treatments (e.g., thinning). At Duke and South Hill,
university collaborators monitored LAI using direct means (destructive harvest and leaf litter); their
data were employed to validate the field optical techniques used in this study.
The fundamental field sampling units are referred to as quadrants and subplots (Figure 4.6). A
quadrant was a 100-
100-m grid with five 100-m east-west TRAC sampling transects and five
interspersed transects for hemispherical photography (lines A-E). The TRAC transects were spaced
at 20-m intervals (north-south), as were the interleaved hemispherical photography sampling
transects. A subplot consisted of two 50-m transects intersecting at the 25-m center point. The two
¥
Table 4.1
Location Summary for Six Validation Sites in the Albemarle-Pamlico Basin
Location
(lat., long.)
Elevation
(m)
Physiographic
Region
Site
State
Ownership
Area
Appomattox
VA
37.219, -78.879
165-215
Piedmont
Private
1200 m
2
(144 ha)
Duke FACE
NC
35.975, -79.094
165-180
Piedmont
Private
1200 m
2
(144 ha)
Fairystone
VA
36.772, -80.093
395-490
Upper Piedmont
State
1200 m
2
(144 ha)
Hertford
NC
36.383, -77.001
8-10
Coastal Plain
Private
1200 m
2
(144 ha)
South Hill
VA
36.681, -77.994
90
Piedmont
Private
1200 m
2
(144 ha)
Umstead
NC
35.854, -78.755
100-125
Piedmont
State
1200 m
2
(144 ha)
 
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