## 2 Experimental Set Up  
### Datasets  
Several kinds of multispectral satellite data have been used for flood mapping, such as Advanced Very High Resolution Radiometer (AVHRR), MODIS, and Landsat TM/ETM+ data. However, most of these data do not have high spatial and temporal resolution at the same time [23]. This has limited their ability to map inundation, which changes complicatedly and rapidly over time. For example, AVHRR and MODIS have a frequent revisiting cycle, which can be even shorter than 1 day. Their high temporal resolution makes them useful for monitoring environmental changes, while the spatial resolution of AVHRR and MODIS is 1 km and 250 m, respectively, which is coarse. Only general extent, not accurate results, can be obtained using these data for flood mapping. On the contrary, Landsat TM/ETM+ data have a middle-to-high spatial resolution of 30 m, but the observation is repeated every 16 days, which cannot satisfy the needs for timely response.  
On 6 September 2008, two optical satellites named HJ-1A/B (short for HuanJing-1A/B), also known as the Chinese Environment and Disaster Monitoring and Forecasting Small Satellite Constellation, were launched in China. The data can be downloaded from the website ([[http://www.cresda.com/](http://www.cresda.com/)]([http://www.cresda.com/](http://www.cresda.com/))) free of charge, and have been successfully applied in several applications such as land mapping, yield prediction, and environment assessment. The two satellites were equipped with CCD cameras, which take multispectral images on the earth surface with a spatial resolution of 30 m. For each satellite, the time interval is 4 days. The constellation of the two satellites theoretically has a higher revisiting frequency of 2 days. With both the advantages of spatial and temporal resolution, HJ-1A/B satellites are regarded as an effective tool for monitoring and post-flood assessment [24].  
The recently emerged geostationary satellite GaoFen-4 (GF-4) also has a high application value in rapid assessment and emergency response of floods [25]. Due to its optical geostationary orbit, GF-4 shows a better performance in time resolution over other satellites. It is equipped with a camera for visible, near infrared and middle-wavelength infrared spectra. The spatial resolution is 50 m. To the best of our knowledge, research work using GF-4 imagery is limited, as it was only launched on 29 December 2015, and officially put in use on 13 June 2016. In this work, we also want to explore the potential of the multispectral GF-4 PMS data in flood mapping. The main parameters of the HJ-1A/B CCD data and GF-4 PMS data are listed in Tables 1 and 2. Slightly different from the HJ-1A/B CCD data, the GF-4 PMS data have an additional panchromatic band.  
As can be seen, the HJ-1A/B CCD data and the GF-4 PMS data show balanced abilities in spatial and temporal resolutions. Due to the limitations stated above in other multispectral data, we decide to  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Satellite Sensor** & **Band No.** & **Spectral Range (um)** & **Spatial Resolution (m)** & **Revisiting Time** \\ \hline  & 1 & 0.43–0.52 & & \\ HJ-1A/B CCD & 2 & 0.52–0.60 & & \\  & 3 & 0.63–0.69 & & \\  & 4 & 0.76–0.90 & & \\ \hline \end{tabular}
\end{table}
Table 1: Technical parameters of the HJ-1A/B CCD data.  
utilise these two datasets for verifying the methods, and for understanding more about the potential of these two datasets in flood mapping as well, as they are not so commonly used as MODIS or Landsat data. Certainly, another reason why the HJ-1A/B CCD and GF-4 PMS data are chosen is because of their free access.