This study was designed (1) to measure differences in the
density of trees in paddy fields in the whole Northeastern
Region and (2) to test several hypotheses about factors
influencing variations in density. Satellite images and GIS
data from several different sources were used in conducting
this analysis as summarized in Table 1.
Measuring Spatial Variations in Density of Trees
in Paddy Fields in Different Parts of the Region
Northeast Thailand covers an area of 168,854 square
kilometers (Fig. 1). We surveyed the density of trees in
various parts of the region using IKONOS satellite images
available through Point Asia. Point Asia contains a collection of high resolution satellite imagery (IKONOS).
IKONOS collects black and white (panchromatic) images
with 0.82 m resolution and multispectral imagery with 4 m
resolution. Imagery from the panchromatic and multispectral sensors can be merged to create 0.82 m color
imagery (pen-sharpened) (Geo-Eye website 2012). It is
well known that one can recognize and identify individual
tree crowns (i.e., all of the above-ground parts, including
lateral stems, leaves and reproductive structures that extend
from the plant’s trunk or main stem) in IKONOS satellite
images (Gougeon and Leckie 2006; Kubo and Muramoto
2008; Grish Kumar and Padmaja 2012). The resolution is
sufficiently high to allow the identification of the boundaries of paddy fields and the location of trees within these
fields using the naked eye. At the time the study was
undertaken, Point Asia included high resolution coverage
of most of Thailand, whereas Google Earth only coveredabout 20 % of the country (Udompitisap and Duangvichitkun 2006). A limitation of Point Asia images is
that they are not date stamped making it difficult to know
the exact year and season when the images were taken but,
according to Point Asia Dot Com Co., Ltd., all of the
images were taken in 2003–2007. The images employed in
this study were accessed from Point Asia (http://www.
pointasia.com) between 1 April and 31 August 2011.
The survey was conducted in the following steps: