2.2.1. Data Source

In total, 120 scenes of cloud-free Landsat images in the years1986, 1988, 1991, 1994, 1996, 1999, 2006, 2013, 2015 and 2019, were selected for fish pond delineation. Landsat-5 TM imagery were adopted from the 1980s to the 2000s, and Landsat-8 OLI imagery were used for the 2010s. Both of the two datasets have the characteristics of multiple bands and high spectral resolution, equipped with rich information and possessed 30 m spatial resolution [17]. The near-infrared band is extremely sensitive to vegetation and water bodies. Preprocessing processes include radiation correction, geometric correction, mosaic and extraction by GBA boundary were performed by the Landsat images preprocessing module provided by ENVI 5.3.

Because different regions have different degrees of urbanization. Dividing the entire study area of GBA into different sub-regions can enable a better understanding of the degrees of urbanization and their impact on pond variations in different regions. Therefore, to facilitate further analysis, the 9 prefectural cities and Hongkong Special Administrative Region (since the fish ponds in the Macau Special Administrative Region has disappeared for years, it was noted included in the study) were further divided into 25 research units (Figure 2). They included 5 units in the prefectural city of Foshan, namely, (1) Sanshui, (2) Nanhai, (3) Shunde, (4) Gaoming, and (5) Chancheng districts. Besides, 4 units are located in the city of Guangzhou, namely, (6) Panyu, (7) Baiyun and Huadu districts (combined as Unit A), (8) Nansha, (9) Yuexiu, Liwan, Haizhu, Tianhe, Huangpu, Zengcheng and Conghua districts (Combined as Unit B). The other 5 individual prefectural cities (i.e., Dongguan (10), Huizhou (11), Shenzhen (12), Zhuhai (13), Zhongshan (14)) and Hong Kong SAR (15) were set as another 6 units. Then, 4 units in the prefectural city of Zhaoqing: (16) Gaoyao district, (17) Sihui County, (18) Duanzhou district and Dinghu district (combined as Unit C), (19) Deqing County, Huaiji County, Fengkai County, Guangning County (combined as Unit D). Further, there are 6 units in the prefectural city of Jiangmen: (20) Heshan city, (21) Xinhui district, (22) Taishan city, (23) Jianghai district, (24) Pengjiang district, (25) Kaiping city and Enping city (combined as Unit E).

**Figure 2.** The location of 25 research units divided.

To ensure image classification accuracy, field investigations to collect fish ponds' spectral information using portable ASD FieldSpec were conducted in cities such as Foshan and Guangzhou where fish ponds are commonly located (Figure 3). The field-collected spectrum curves of fish ponds were added as part of the spectral library for classification. The accuracy assessment was proposed by building the confusion matrix comparing the extraction results and the accuracy assessment samples we designed based on Landsat images, combined with Google Earth historical images, field survey data and historical land use data, etc. Then, the Kappa coefficient calculated form confusion matrix was adopted to examine the accuracy of the classification results (Figure 4).

**Figure 3.** Photos of field investigations for the collection of ponds' spectral information. The photos were taken by the authors.

**Figure 4.** Flowchart for image classification and analysis.
