## 2 Datasets and Methods
### Datasets
#### 2.2.2 Training and Validation Samples  
The quality of samples is directly related to the accuracy of wetland information extraction, so pure typical and representative pixels should be selected as samples. The training and validation samples in this study are mainly derived from two methods: field survey and visual interpretation with Google Earth high-resolution optical remote sensing images, as well as GaoFen-2 (GF-2) panchromatic and multispectral fusion images with nadir pixel resolution of 0.8 m. In November 2020, our research group conducted a detailed field survey of the YRD National Nature Reserve, using Global Position System real time kinematic (GPS RTK) measurements to locate, record, and take photos of different wetland distributions and wetland types, as shown in Figure 2. The above two categories of data constitute sample data and validation data, which are used to establish classifiers and verify accuracy. Table 3 lists the per-class numbers for the classification training and validation samples (a total of 219,269 samples). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **GF-3** & **OHS** \\ \hline Imaging time & 13 March 2019 & 23 March 2020 \\ Product type & Single Look Complex & STANDARD \\ Nominal resolution (m) & 8 & 10 \\ Product level & 1 & L1B \\ Satellite ID & GF-3 & OHS 2-D \\ Sensor & SAR & CMOSMSS \\ Band number & 4 & 32 \\ Band & 5.6 cm (C) & 400–1000 nm (VIS+NIR) \\ Polarization mode & HH+HV+VH+VV & — \\ Cloud percent (\%) & — & 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of GF-3 and OHS data used in this study. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Band Name** & **OHS** & **Landsat-8 OLI** & **Sentinel-2** \\ \hline Green & 566 nm & & \\
580 nm & 561 nm & & 560 nm \\ \hline Red & 670 nm & 655 nm & 664 nm \\ \hline NIR & 806 nm & & \\
820 nm & 865 nm & & 833 nm \\  & 880 nm & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of OHS bands with Landsat-8 OLI and Sentinel-2. According to the investigation of wetlands in the YRD, the main wetland types in the wetland are _Tamarix chinensis_, _Suada salsa_, _Phragmites australis_, and invasive species _Spartina alterniflora_[7; 37; 41]. * _Tamarix chinensis_ is a kind of tree or shrub with a height of 3-6 m. It is popular on river alluvial plain, seashore, beachhead, wet saline land, and sandy waseland. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Class**} & \multicolumn{2}{c}{**Number of Training Sample**} & **Number of Validation Sample** \\  & **(Points)** & **(Points)** \\ \hline Saltwater & 59,357 & 50,710 \\ Farmland & 20,783 & 12,162 \\ River & 8673 & 3047 \\ Shrub & 2225 & 2695 \\ Grass & 11,401 & 6863 \\ _Suada salsa_ & 269 & 124 \\ Tidal flat & 18,552 & 22,408 \\ Total & 121,260 & 98,009 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The number of classification training and validation samples per class. Figure 2: Schematic diagram of field survey marks and site photos for YRD wetland types in November 2020. Green points represent the GPS RTK locations. Note that the RTK points are sampled at a distance of approximately 5 to 10 m, so there are many points that overlap and are not fully visible in this figure. (**a**) Tidal Flat, (**b**) _Suada sals_ (**c**) _Tamarix chinensis_, (**d**) _Spartina alterniflora_ (**e**) _Phragmites australis_ (**f**) Farmland. * _Sueda salsa_ is an annual herb that grows up to 1 m tall, growing in the seashore, wasteland, ditch shore, the edge of the field, and other saline soil. * _Phragmites australis_ (commonly known as reed) is a tall aquatic or wet perennial grass, up to 1-3 m tall. It does not grow in forest habitats but in various open waterborne areas, such as rivers, lakes, ponds, watercourses, and lowland wetlands. * _Spartina alterniflora_ grows best on muddy beaches in estuaries.