2.2 Data sources
This study used three different spatial scale survey methods to monitor
ephemeral gully: satellite remote sensing monitoring, unmanned aerial
vehicle (UAV) monitoring, and field investigations (Fig. 2). The remote
sensing image data included 0.50 m high-resolution satellite images
(digital orthophoto map, DOM) in 2009, 2012, and 2018 and 0.50 m
high-resolution DOM images by UAV in 2021 (11,720 images). A DEM with
0.15 m resolution and DOM with 0.50 m resolution were created using
Agisoft Metashape (Windows1.7.3, Agisoft, Russia). The 1:10,000 DEM data
used in this study were obtained from the Shaanxi Geomatics Center of
the Ministry of Natural Resources of China, and the 0.15 m resolution
DEM data were obtained by the authors using UAV (Table 1). In order to
avoid the influence of different DEM resolutions on the extraction
accuracy of ephemeral gully morphological parameters, we changed the
resolution of 0.15 m to 5 m by resampling in ArcGIS software (ArcGIS
10.2, Esri, U.S.A). The remote sensing and UAV images were obtained from
April to June each year when the vegetation was not yet lush and thus
had no significant impact on ephemeral gully recognition.
Table 1 Basic information on the DOM and DEM data used in this study.