What is NOT one of the nine essential elements that all maps need?

The essential elements that all maps need are fundamental components that provide crucial information for understanding the map's content and orientation. The title, for example, indicates what the map is about, helping users quickly grasp its primary focus. The compass rose shows direction, which is vital for navigation and understanding the map's orientation in relation to north, south, east, and west. The scale represents the relationship between distance on the map and the actual distance it represents, allowing users to measure real-world distances accurately.

Population density, while informative and important in the context of certain thematic maps, is not a basic requirement for all maps. It is a specific piece of information that may be included in maps depicting demographics or urban planning but does not belong to the core framework that defines what all maps must contain. Therefore, it is correct to identify population density as not being one of the nine essential elements that all maps need.