maize, medium maize, and late maize. Early seasoned and
early-matured maize, mid-seasoned, and medium-matured
maize, late-seasoned, and late-matured maize were classified
as early, medium, and late maize, respectively (Carsky
et al.,
2001). This subdivision was merely for internal classification
to map the overall maize cover accurately and had nothing to
do with actual definition of early, medium, and late maturity
of the crop. Spectral values alone were not sufficient in order
to map maize precisely; consequently additional parameters
were devised to eliminate confusion in separating these crops.
These parameters included standard deviations,
NDVI
, and
simple ratios. The final decision rules developed are shown
in Table 1.
Classification Rules Implementation
First, the late-maize rule set was executed at level 2 (Figure
8). Bare soil near houses, having spectral characteristics simi-
lar to that of late-maize, was included in late-maize. Subse-
quently, early-maize rule set was implemented on the remain-
ing unclassified image objects of level 2. Some small image
objects, i.e., trees, were also included in early-maize class.
Finally, medium-maize rule set was applied to the remaining
unclassified objects at level 2.
In the next step, early-maize, medium-maize, and late-
maize were combined into one class namely “combined
Figure 7. Spectral signatures of remaining crops. X-axis NIR, Red and Green channels data is plotted whereas mean reflection response
comprising 16-bit data are plotted on Y-axis.
T
able
1. M
aize
(L
ate
, M
edium
,
and
E
arly
) C
lass
D
escription
in
T
rimble
e
C
ognition
®
Late Maize
Medium Maize
Early Maize
Mean NIR >= 15000
Mean NIR <= 29500
Mean Red >= 28000
Mean Red <= 43000
Mean Green >= 27000
NDVI >= -0.3
NDVI <= 0
Green/Red <= 1
NIR/Green <= 1
SD Green >= 1850
Mean NIR >= 15000
Mean NIR <= 28500
Mean Red >=19000
Mean Green >= 23000
Mean Green <= 30000
NDVI >= -0.3
NDVI <= 0.02
Green/Red <= 1
SD NIR >= 1200
SD Red >= 1500
SD Green >= 1700
Mean NIR >= 15000
Mean NIR <= 30000
Mean Red >= 19000
Mean Red <= 27000
Mean Green >= 23000
Mean Green <= 31000
NDVI >= -0.2
NDVI <= 0.17
Green/Red >= 1
SD NIR>= 1200
SD Red>= 1500
SD Green >= 1750
Figure 8. Implementing early maize, medium maize, and late
maize rule set.
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
August 2015
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