Sampling
A grid composed of 1*1 km cells was overlaid on an image
of Northeast Thailand for the area from 14000N–100500E to
18300N–105400E. This grid covers all of Northeast Thailand
with a total of 176,669 grid cells (there are 501 cells on the
north–south axis and 521 cells on the east–west axis). Initially,
350 of these grid cells were randomly selected but, after
checking the Point Asia images, 147 cells had to be excluded
from the survey because they were found to be unsuitable for
analysis. Of these, 88 grid cells had no satellite images or the
images could not be utilized because of too much cloud cover
(25.1 % of the initial sample of grid cells); 59 grid cells
(16.9 % of the initial sample) had satellite images but contained either no paddy fields or such a small area of paddy
fields as to be unusable. The images for the 203 remaining gridcells (0.1 % of the total number of grid cells) were analyzed in
this survey (Fig. 2).
Identifying Paddy Fields in the Grid Cells
Within each of the sample grid cells (1 9 1 km), the
boundaries of paddy fields were identified by the naked eye
of the analyst and the area of the paddy fields was calculated using Arc View version 9.1 (ESRI) geographical
information software.
Calculating the Density of Trees in Paddy Fields
All of the tree canopies inside the boundaries of the
paddy fields within each grid cell were counted using the
naked eye of the analyst. In those cases, when there
were several trees forming a single cluster, and it was
not possible to count each tree individually due to
insufficient resolution, the whole cluster was counted as
one tree. In many cases, these clusters are clumps of
bamboo while, in other cases, they are composed of
several small trees growing closely together on top of a
termite mound, so are functionally equivalent to a single
tree. An example of counting trees in paddy fields is
shown in Fig. 3. Once the total number of trees in all of
the paddy fields within a grid cell was established, then
the density of trees was calculated with the following
formula using Arc View:
The mean density of trees in paddy fields number/ha ð Þ
¼ total number of trees in paddy fields/area
of paddy fields ha ð Þ: