It is commonly recognized that navigating in three-dimensional spaces can be a complicated task, primarily when working with "objects" that have natural orientations such as terrains. To resolve this problem, we limited user navigation to two types of rotations. This strategy prevents the user from becoming "lost" while "flying" through the environment and feel trapped by the tool. Also, based on the fact that there is always a center of interest, user navigation is limited to the sphere surface whit its center located in this interest area. This paper presents an overview of how this 3D navigation problem is resituated in the context of terrain explorations. Our goal is to help the user navigate 3D terrains by adapting the conventional Virtual Sphere navigation approach to the particular case of "terrain" explorations. We argue that the traditional schemes are too generic for some specific 3D uses, and we propose exploration constraints that are better suited to these particular tasks. We already used this approach, with optimal results, in different contexts, like hierarchical visualization, earthquakes visualization, earthquake animation and 3D wind visualization in Costa Rica.

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