The first sensor collects a first group of first sensor readings of first fixed size dependent on a range of a first sensor and safeguarding requirements of a vehicle. A second sensor collects a second group of second sensor readings of second fixed size dependent on a range of a second sensor and the safeguarding requirements. A reference frame manager references the first group and the second group of readings to a vehicle coordinate reference frame. An integration module establishes a composite grid referenced to the ground based on a spatially aligned integration of the readings of the first group and the second group. A update module refreshes the composite grid upon material movement of the vehicle such that the global occupancy of each cell in the composite grid varies in accordance with the particular location of the vehicle on the terrain.