The low altitude aerial photograph is used for the investigation and the research in various fields as well as satellite photograph and general aerial photograph, and it is used in the river fields today. This photography method includes several kinds and it is necessary to select the best method for the riverbeds gravel investigation. However, very few attempts have been made to examine these several methods at simultaneity and to verify using photographic measurement method. Therefore, we have taken photographs of several fields and have analyzed digital images using photographic measurement technique of CAD software. As a result, we have concluded that we can separate 10 mm gravel of digital image at altitude 10 m and this method is effective for riverbeds gravel investigation, especially investigation of large gravel. In this paper, we describe accuracy of digital image and practicality of low altitude aerial photograph.

Key wordsFlow altitude aerial photograph, riverbed gravel, polygon analysis, CAD, photographic measurement