Geoscience Reference
In-Depth Information
N
Quadrant
100 m
Sub
L1_50
L2_0
S
L1_0
L1_100
20 m
A_10
A_90
L2_0
L2_100
L3_0
L3_100
L1_0
L2_50
L4_0
L4_100
Hemi
TRAC
L5_0
L5_100
E_90
E_10
Figure 4.6
Quadrant and subplot designs used in the Albemarle-Pamlico Basin study area.
transects were oriented at 45˚ and 135˚ to provide flexibility in capturing TRAC measurements
during favorable morning and afternoon solar zenith angles.
Quadrants were designed to approximate an ETM
3 pixel window. Subplots were designed
to increase sample site density and were selected on the basis of ETM
3
¥
+
NDVI values to sample
over the entire range of local variability. Quadrants and subplots were geographically located on
each LAI validation site using real-time (satellite) differentially corrected GPS to a horizontal
accuracy of
+
1 m. TRAC transects were marked every 10 m with a labeled, 46-cm wooden stake.
The stakes were used in TRAC measurements as walking-pace and distance markers. Hemispherical
photography transects were staked and marked at the 10-, 30-, 50-, 70-, and 90-m locations.
Hemispherical photographs were taken at these sampling points.
The APB quadrant design was similar to a measurement design used in a Siberian LAI study
in the coniferous forest of Krasnoyarsk, Russia (Leblanc et al., 2002). Here, each validation site
had a minimum of one quadrant. Multiple quadrants at Fairystone were established across a 1200-
±
¥
1200-m oak-hickory forest delineated on a georeferenced ETM
image to approximate a MODIS
+
pixel (1 km
), with a 100-m perimeter buffer to partially address spatial misregistration of a MODIS
pixel (Figure 4.7). The stand was quartered into 600-
2
600-m units. The northwest corner of a
LAI sampling quadrant was assigned within each quarter block using a random number generator.
A SRS design was used to select ground reference data spanning the entire range of LAI-NDVI
values. Fairystone sites were stratified based on a NDVI surface map calculated from July 2001
ETM
¥
imagery. Analysis of the resulting histogram allowed for the identification of pixels beyond
+
±
1 standard deviation. From these high/low NDVI regions, eight locations (four high, four low)
were randomly selected from each of the four 600-
600-m units. Subplots were established at
these points to sample high or low and midrange NDVI regions within each of the four quadrants.
¥
4.3.2
Biometric Mensuration
The measurement of crown closure was included in quadrant sampling to establish the rela-
tionship between LAI and NDVI. Wulder et al. (1998) found that the inclusion of this textural
information strengthened the LAI:NDVI relationship, thus increasing the accuracy of modeled LAI
estimates. Crown closure was estimated directly using two field-based techniques: the vertical tube
