The planar fit technique is a coordinate rotation scheme widely used for tower-based eddy covariance measurements. Applicability of this technique was examined with respect to the geometry of the mean stream surface. Analyses based on a 3-year eddy-covariance dataset obtained over flat terrain of a paddy field revealed that the mean streamline was upward at almost all wind directions possibly because of flow distortion by the instruments. Fitting a single plane to the concave stream surface led to miscalculations in the offset and tilt angle of the mean streamline. Consequently, the half-hourly mean vertical wind velocity and streamwise momentum flux demonstrated inconsistent distributions in planar fit coordinates. In contrast, they were consistent under a modified planar fit technique, the sectorial planar fit, in which the mean stream plane was determined at each wind sector with a center angle of 30°. The planar fit technique is not applicable when the mean stream surface is not approximated by a plane. In such cases, the sectorial planar fit can be a practical alternative to the ordinary planar fit.