Abstract
In Japan, Ministry of Agriculture, Forestry and Fishery (MAFF) is planning to modify of determination system of crops planting areas. MAFF expected high accurate results and work saving procedures using remote sensing data. We determine the paddy field areas in selected regions with very high accuracy using RADARSAT at transplanting season combined with Landsat TM data of past observation. The method thus developed in this study is very useful not only in estimating the crop area very early in the growing season also it can be used at all weather conditions.
We also studyed the relationship between rice growth and backscatter coefficients of RADARSAT/SAR.
1. Introduction
Rice is one of the important crops in Japan. Therefore it is vital to monitor the growth conditions of rice and estimate the area of paddy field as accurately as possible. This also helps design a good food supply plan. Accurate estimation of rice fields, using Landsat TM data (Okamoto, et al. 1996) as well as using ERS-1 data (Panigrahy, et al. 1997) have been well studied. Kurosu, et al. (1997) has also published, recently, of monitoring of rice fields using ERS-1 data. However, the size of paddy fields in Japan is small and there is mixture of crops in the fields with many land use covers. Further more, Japan is located in monsoon area and there are only one or two optical sensor data sets become available during the planting terms. It is virtually impossible to acquire optical sensor data every year during the rice planting seasons.
In Japan, paddy fields are flooded during the rice-planting season and when we use RADARSAT data, water covered areas; paddy fields and mountain shades have low backscatter coefficients. This makes the water-covered areas, paddy fields and mountain shades easy to distinguish then from other land covers. The defaults of SAR data is single banded and contains speckle noise. On the other hand, additional use of Landsat TM data combined with RADARSAT, water covered areas could be distinguished from the mountain shade areas and water covered areas.
The objective of this study, is to estimate the paddy field area annually and accurately mainly using the RADASAT data by monitoring the change in backscatter coefficients in the rice fields. First, we tested some filtering methods to reduce the speckle noise of RADARSAT data, as it is possible to be acquiring RADASAT data sets at any time by changing the sensor angles. We also combined the classified image data from Landsat TM with RADARSAT data, to estimate the paddy field areas with high precision, under all weather conditions.
2. Test Site and using Data
Test site for monitoring paddy field in this study was Kantoh plain Tochigi, Japan. Test site for estimating paddy field area was Ishikari plain Hokkaido (Fig. 1). Tochigi area is located in the center of Japan, while Hokkaido is located in the northern part of the Island.
There are 18 districts in the test site, Ishikari plain where paddy fields occupy large areas. All paddy fields are flooded in planting season and after planting, from the end of May to middle of June. Two Landsat TM data sets were used to select the paddy field candidates. Another TM data set was used to compare the paddy field estimated from this method. RADARSAT data set was used to estimate paddy field areas in the paddy field candidates made from two Landsat TM.
Fig. 1 Test site location
Three RADARSAT data sets were used to monitor the paddy fields and three Landsat TM combined with one RADARSAT data sets were used to estimate the paddy field areas (Table 1).
In Tochigi area, paddy field is flooded in early May, and rice is also planted during this season. Therefore, RADASAT data sets obtained on 17 May, when rice was in its early growth stage of about 15 - 20 cm height and on 27 June, when rice was grown to 50 - 60 cm height were used. Ground survey provided the information about the status of rice growth in the field. The other RADASAT data used was collected on 28, July, just prior to flowering, when rice was grown up to 70 - 80 cm height. ,In Hokkaido area, paddy field is flooded by the end of May. Planting season in northern area is later than in the center of Japan, Tochigi area. Rice in this area is in its early growth stage by 5 and 17 of June.
Table 1. Landsat TM and RADARSAT data sets.
| Sensor |
Observation date |
Use |
| Landsat TM |
12-Jun-96 |
Removing areas of open water, mountainous shadows and selecting paddy field candidates |
| Landsat TM |
8-Jul-93 |
| Landsat TM |
17-Jun-97 |
Verifying paddy field |
| RADARSAT |
5-Jun-97 |
Estimating paddy field in the paddy field candidates |
| RADARSAT |
17-May-98 |
transforming digital number of path image into backscatter coefficient |
| RADARSAT |
27-Jun-98 |
| RADARSAT |
28-Jul-98 |
| Landsat TM |
21-May-87 |
Verifying land use |
| Landsat TM |
24-Jul-93 |
