What is NOT a type of landmark?

In the context of vertical navigation, landmarks are crucial for pilots to recognize and use for orientation and navigation. There are several types of landmarks, including positive landmarks, which are easily identifiable features that distinctly mark a location, and linear landmarks, which are elongated features such as rivers or highways that can guide navigation along a route.

The term "certain" does not refer to an established category of landmark types in this context. It does not fit within the established classifications of landmarks used for navigation. Therefore, identifying "certain" as not being a type of landmark is accurate, as it lacks a clear definition or standard usage in navigation terminology. Instead, both positive and linear are recognized categories that hold significant importance in aiding navigators.