Spatial user interfaces that help people navigate often focus on turn-by-turn instructions, ignoring how they may help incidental learning of spatial knowledge. Drawing on theories and findings from the area of spatial cognition, the current paper aims to understand how turn-by-turn instructions and relative location updates can help incidental learning of spatial (route and survey) knowledge. A user study was conducted as people used map-based and video-based spatial interfaces to navigate to different locations in an indoor environment using turn-by-turn directions and relative location updates. Consistent with existing literature, we found that providing only turn-by-turn directions was in general not effective for helping people to acquire spatial knowledge as relative location updates, but map-based interfaces were in general better for incidental learning of survey knowledge while video-based interfaces were better for route knowledge. Our result suggested that relative location updates encourage active processing of spatial information, which allows better incidental learning of spatial knowledge. We discussed the implications of our results to designs trade-offs in navigation interfaces that facilitate learning of spatial knowledge.