Page 107 - Contributed Paper Session (CPS) - Volume 7

CPS2042 Anna Christine D. et al.
The stratification in this study was conducted prior to the selection of
meshes to improve statistical efficiency and lower fieldwork costs. The first
stratum consisted of meshes that both IRRI and ESA maps (IRRI+GlobCover
stratum) identified as paddy rice area, considered to be the most likely to
contain paddy rice. The second stratum, considered a medium probability
5
stratum , consisted of meshes that were only identified as rice by the IRRI area
map (IRRI stratum) but not by the ESA map.
The third stratum is the low probability stratum, identified as rice by ESA’s
map (GlobCover stratum) but not by IRRI’s map. The final stratum consists of
all remaining areas where presumably no rice is grown as indicated by both
IRRI and ESA maps, henceforth referred to as the Other stratum.
In the first sampling stage, a stratified random sample of 120 meshes was
selected for each pilot province . The number of selected meshes was higher
6
in the stratum where the expected likelihood of finding rice growing plots is
highest (Stratum 1), and lower in areas with low (Stratum 3) or no likelihood
(Stratum 4) of finding rice growing plots. A ground-truthing field operation
was conducted to verify whether rice was planted in any plots within the
boundaries of each sample mesh. Only sample meshes with rice were
enumerated for eligibility to be selected for crop cutting in the second
sampling stage.
Systematic random sampling was then used to select a sample of four
plots per mesh from the list of plots that met the selection criterion. This
involved calculating a sampling interval, which was used to systematically
select the sample plots from the ordered list, following a random start. The
selection of four plots was driven by the need to ensure sufficient sample size
within a mesh to capture variability in rice yields across plots and budgetary
constraints.
At the third sampling stage, a random point was selected within each
sample plot to identify a 2.5 m x 2.5 m crop-cutting subplot. This was
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followed by the measurement of area planted in rice in sample plots within
each sample mesh, which was also used as component for the sample
weighting procedure. Two sources of objective measurements for the area of
the sample rice plots were used: (i) unmodified tracks that are based on the
boundaries of the plot recorded by the enumerators using a handheld GPS

5 the resolution of the IRRI map obtained from MODIS is better and more recent than the ESA
map obtained from ENVISAT
6 The total number of meshes was based on the expected number of rice plots to be found and
interviewed in each stratum using d ata from pretests and the available budget for the pilot
project.
7 Crop cutting is a method wherein a small portion of a randomly selected plot, henceforth
referred to as a subplot, is harvested, threshed, dried, and weighed to obtain objective yield
estimates (Huddleston 1978).
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