A recent navigation apparatus can provide not only a two dimensional map but also a bird's-eye view, which makes a present position more discernible. Further, the navigation apparatus is required to have another function such as recording user's favorite songs. Thus, in the navigation apparatus, a storage medium for storing map data is required to have more data volume. To that end, a CD-ROM or DVD-ROM is generally replaced by a hard disk drive (HDD).
A hard disk drive typically contains a magnetic disk for storing data and a magnetic head for writing and reading data with the magnetic disk. With an access from a control device, a magnetic disk rotates, thereby generating wind force of air streams. The wind force causes the magnetic head to separate from the magnetic disk with a minute gap (about 0.1 micrometer); namely, the magnetic head is in a lifting state in which the magnetic head is lifting or floating from the magnetic disk. In such a lifting state, the magnetic head performs writing or reading data with the magnetic disk.
Thus, a hard disk drive has a structure in which a magnetic head is lifted from a magnetic disk with a minute gap using wind force resulting from rotation of the magnetic disk. When writing or reading data is performed based on an access from a control device, a vibration or impact may be exerted. In such a case, the magnetic head may abut on the magnetic disk to thereby damage the magnetic disk or destroy data.
Patent document 1 describes a technology to record, in map data, certain places where vibration or impact may be exerted to a vehicle passing therethrough. Thus, if a vehicle passes through the certain place, an access to the hard disk drive is prohibited.                Patent document 1: JP-2004-264115 A        
In short, in Patent document 1, when passing through the certain place where vibration or impact occurs, an access to the hard disk drive is prohibited. As explained above, a hard disk drive has a structure in which a magnetic head is lifted from a magnetic disk using wind force of air streams resulting from rotation of the magnetic disk. Therefore, if the wind force produced by rotation of the magnetic disk is small, the magnetic head may not be floated and the magnetic head may continue contacting the magnetic disk.
The wind force of air streams produced by the rotation of the magnetic disk changes depending on a density of the air. That is, even if the stream has the same speed, a resulting wind force is increased as the density of the air is increased; a resulting wind force is decreased as the density of the air is decreased. The density of the air is influenced by an atmospheric pressure and the atmospheric pressures is fundamentally influenced by an altitude. For example, a road may run on a high ground (e.g., several thousands meters above sea-level). In such a high ground, an atmospheric pressure is low; therefore the density of the air is low. The wind power of the air stream produced by the rotation of the magnetic disk is thereby decreased; therefore, the magnetic head may not be maintained in the lifting state against the magnetic disk. In this case, even if there is neither vibration nor impact, the magnetic head and the magnetic disk may contact each other to thereby damage the magnetic disk. That is, when running a high ground where an atmospheric pressure is low, the technology of Patent document 1 cannot solve such a problem.