Estimation of Reservoir Submerging Losses Using CIR Aerial Photographs: Example of the Ertan Hydropower Station on the Yalong River in Southwest China

Abstract

A geotechnical project, which used remotely sensed and other spatial data to estimate reservoir submergence losses, was carried out in 1982 at the Ertan hydropower station on the Yalong River in Sichuan province, southwest China. To meet the needs for use of data from spaceborne remote sensing to estimate the losses, color-infrared (CIR) aerial photographs were used as the first step in a series of experiments. An orthophotomap and a land-cover and land-use map of the submerged region have been completed using 1:45 000 and 1:24 000-scale CIR aerial photographs taken in the winter and spring of 1981. After some typical on-site sampling in the region to be submerged, as well as synthetic analysis on the aerial photographs, a three-level classification system has been developed, with 25 categories of potentially submerged objects classified by use of the maps. Each level of the classification system is decided by the needs of the project design for investigating submergence losses, the resolution of the aerial photographs, and the minimum size of the mapping unit. The length or area of each category on the land-cover and land-use map (1:20 000-scale) is measured using a manual tracking digitizer. The root-mean-square (rms) accuracy standard was adopted to check the accuracy of the estimate. The accuracy includes the classification and position accuracy of the land-cover and land-use map, and the measuring accuracy of the polygons on the maps. Among the 25 classification categories, the difference between the statistical acreage of cultivated land determined from the estimation method using CIR aerial photographs and from the conventional method of on-site investigation by surveyors was only 3.1%. This project has provided not only the data needed for the design of the hydropower station, but also a data base for use in future experiments using Landsat data.