Enabling natural and intuitive communication with robots calls for the design of intelligent user interfaces. As robots are introduced into applications with novice users, the information obtained from such users may not always be reliable. This paper describes a user interface approach to process and correct intended paths for robot navigation as sketched by users on a touchscreen. Our approach demonstrates that by processing video frames from an overhead camera and by using composite Bézier curves to interpolate smooth paths from a small set of significant points, low-resolution occupancy grid maps (OGMs) with numeric potential fields can be continuously updated to correct unsafe user-drawn paths at interactive speeds. The approach generates sufficiently complex paths that appear to bend around static and dynamic obstacles. The results of an evaluation study show that our approach captures the user intent while relieving the user from being concerned about her path-drawing abilities.