This paper discusses a sketch interface that can be used to guide a mobile robot along a specified path in its unfamiliar place. With the sketch interface, the user draws a rough map to give navigation tasks to robots. Because sketched maps often suffer from various inaccuracies and large errors in landmarks, we discuss what kinds of uncertainties in the rough maps would mainly have effects on navigating a robot. The effects of such inaccuracies on robot navigation are analyzed in simulated environments. A quantitative navigability measure of rough maps is then developed based on the analysis. Experimental results are also presented for validating the navigability measure.