Large scale particle image velocimetry (LSPIV) is an efficient and powerful tool to measure river surface flow. In LSPIV oblique angled video images taken from a river bank or a bridge are used to measure two-dimensional surface velocity distributions. In the original LSPIV algorithm, water surface is assumed to be in a horizontal plane. This as-sumption holds true for a mild-slope channel, however, it can be a cause of error in the image transformation when a steep flow is viewed from an oblique angle. Hence, in order to generalize the LSPIV, a water surface equation is in-troduced to the image correction algorithm, which allows the image analysis on a tilted surface plane. In addition, a new formula for verifying velocity resolution is derived using an image transformation equation. As an application of LSPIV, large scale recirculating flows between the groins installed in the middle reach of the Nagara River were measured with the aid of non-toxic tracer particles. An improved LSPIV was found to be quite useful for analyzing surface flow with a reasonable accuracy.