# Establishing a Geological Knowledge Base for Braided River Deltas Using Google Earth
## 2 Materials and Methods
### Data Acquisition and Processing  
High-resolution satellite imagery covering delta plain, front, and prodelta sedimentary units is extracted using the Google Earth Pro platform (Figure 1) [15]. This study obtained high-resolution satellite imagery through the Google Earth Pro platform, with the image shooting time concentrated from July 2018 to October 2022. During this period, the vegetation coverage is low and the water transparency is high, which is conducive to the clear identification of river channel boundaries and sedimentary units. Straight-line distances are measured in \"Line mode\" via the \"Add path\" tool on orthoimages, while curved distances (defined as bifurcation spacing) are obtained by switching to \"Path mode\", adding anchor points along channel bends, and generating automatic curve lengths. High-definition images are exported to CorelDRAW Graphics Suite 2022 (Version number: 24.1.0.360, released on 15 June 2022) (Figure 2), where sub-channel centerlines are fitted with the bifurcation point as the origin, and the included angle between two channels is measured using the \"angle measurement tool\" (Table 1).  
Figure 1: Part of the image of a braided river delta shown by Google Earth.  
Figure 2: Measurement schematic diagram.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Poyang Lake** & **Channel Width (m)** & **Wavelength (m)** & **Amplitude (m)** & **Spacing (m)** & **Angle (°)** \\ \hline
1 & 1136 & 9133 & 1591 & 4533 & 50.11 \\
2 & 745 & 7265 & 2214 & 25,815 & 38.69 \\
3 & 501 & 9505  
In this study, the morphological data of braided river deltas were manually digitized using Google Earth (resolution of 5-15 m/pixel) and CoreIDRAW software. Affected by the image resolution and the subjectivity of human measurement, there are two main types of errors in the data: positional deviation and measurement repeatability error.  
The limitation in image resolution directly affects the measurement accuracy. According to the research of [PERSON] et al. [30], a manual picking deviation of 10 pixels will lead to an actual positional error of 50-150 m (calculation method: positional error = pixel deviation \(\times\) image resolution). This error has significantly different impacts on parameters at different scales: within the wavelength dataset range of this study (1500-4000 m), a 150 m error accounts for 3-10%; for small-scale amplitude parameters (such as the average amplitude of the Saloum River delta being 680 m), the proportion of the same error is as high as 22% (Table 2). To control the error, the manual picking accuracy in this study is strictly limited to \(\leq\)5 pixels (corresponding to an error of 25-75 m), reducing the positional error by 50% compared to the theoretical maximum value. Nevertheless, small-scale param  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Poyang Lake** & **Channel Width (m)** & **Wavelength (m)** & **Amplitude (m)** & **Spacing (m)** & **Angle (\({}^{\circ}\))** \\ \hline
18 & 245 & 2230 & 436 & 1383 & 45.25 \\
19 & 185 & 934 & 499 & 1658 & 25.67 \\
20 & 191 & 5244 & 1544 & 684 & 6  
eters are still greatly affected. Taking the Saloum River delta as an example, the measured amplitude value may fluctuate by \(\pm\)10-22% due to errors.  
The measurement repeatability error stems from the differences in the subjective judgments of the measurers. In this study, through three independent digitizations of four deltas, the mean absolute deviation of the wavelength measurement is calculated to be 8.7%, and the amplitude deviation is 12.3%, which is in line with the error range of manual digitization (5-15%) proposed by [PERSON] [31]. To reduce this error, three measurements are carried out for key feature points (such as river channel inflection points and tributary intersection points), outliers are removed, and the average value is taken.