Abstract: In the process of sewage outlets investigation, it is often impossible to find hidden sewage outlets under trees and bridges by using orthophoto alone. In order to solve this problem, this paper selects the main stream area of Songhua River, and proposes a hybrid interpretation method using UAV orthophoto, 3D model and water quality inversion parameters. Firstly, the key investigation area is determined by the abnormal information of water quality parameters, and then the large-scale investigation is carried out by orthophoto to mark the suspected sewage outlets. At the same time, the real 3D model constructed by oblique photography is used to interpret the key areas and suspected sewage outlets. This method is conducive to the rapid and efficient search for hidden sewage outlets, and has great significance for tracing the source of river and marine water pollution.

Key words: UAV, sewage discharge outlets, 3D model, water quality inversion, orthophoto