A sextant is indispensable for the determination of boat-position where the sampling or observation is performed for the water pollution research. However, it is difficult to measure angles between terrestrial objects by the sextant within a short time interval and particularly while the boat is moving. Thus, the investigator is so much devoted to the time-consuming measurement by the sextant and the field work has to be finished frequently without satisfaction.
The authors, therefore, attempted to apply the terrestrial photogrammetry to locat the boat-position by using a common-type camera in place of a surveying camera which could not be used under a rough weather condition. The use of the common-type camera for measuring angles yielded considerable errors due to the discrepancy between the focal distance of camera-lens and the picture distance proper to the camera employed. In order to reduce such errors the authors proposed equation (1) for calculating the real picture distance, and it was demonstrated that the angles between objects on the picture could be determined with the errors smaller than ±6'30" provided that the real picture distance was utilized. Equations (2), (3), (4), (5) and (6) are presented for the computation of the error of the boat position which corresponds to the space between the real boat-position and the estimated boat-position. Thereby, the magnitude of the error of boat-position in this photogrammetry with a common-type camera was evaluated to vary from 2m to 39m, with the average of 19m. In other words, it can say that the photogrammetry mentioned above possesses such an accuracy that the boat-position can be determined within a circle smaller than 2mm in radius when plotted on a chart with a scale of 1/10, 000 of the natural size.