Modis Cloud Masking for South east Asia Region
Vivarad PHONEKEO, Preesan RAKWATIN, Lal SAMARAKOON, Ryuzo YOKOYAMA
Asian Center for Research on Remote Sensing (ACRoRS)
Asian Institute of Technology (AIT)
P.O.Box 4, Khlong Luang, Pathumthani 12120
Tel: (66)-2-542-6197 Fax: (66)-2-524-6147
Email: vivarad@ait.ac.th
Combination Threshold
Our study is to develop cloud-masking algorithm applied to MODIS images over South East
Asia region, which could be different from other areas in terms of surface characteristics and
atmospheric conditions.
Clouds are normally characterized by higher reflectance and lower temperature (below 0°C to
20°C) than earth surface. However, very thin cloud has low reflectance and low temperature.
Low altitude cloud (including fog) has high reflectance but not necessarily low temperature.
From these characteristics, multispectral statistic technique based on visible and infrared
threshold is applied.
Our methodology is first randomly select sample areas of cloud, land, and sea by 10 by10 pixels
of MODIS 1km resolution. Then we calculate min, max, mean, and standard deviation of these
selected samples. After that, we applied single band threshold and two-band combination
threshold between emissive band and reflective band to detect the clouds over the land and
clouds over the sea. By visual observation, we found that, the results of two-band combination
threshold are better than one band threshold. Band 4 and band 31 combination is good for detect
cloud over land surface. Band 6 and band 31 combination or band 17 and band 31 combination
are good for detect cloud over sea surface.
1. INTRODUCTION
One of the major problems for land and sea surface observed by MODIS image is the cloud. It
reflects incoming solar radiation back to space and traps the thermal radiation emitted from the
Earth’s surface. The purpose of cloud detection is to identify the presence of cloud in the image
so as to eliminate it from clear pixels. It is very useful for specifying land cover types,
determining sea and land surface temperatures, and etc without cloud.
Furthermore, evaluating annual cloud coverage will benefit the meteorological, Oceanographic,
and Earth resource studies through this region. This information will offer a useful indication for
the other research since the fundamental feature of earth as viewed from space is climate
dominated by a dynamic cloud atmosphere.
