The present invention provides several apparatus and methods for generating a terrain clearance floor envelope for use in a ground proximity warning system. The present invention includes a processor that defines a terrain clearance floor envelope (10) having at least two boundaries (14, 22). In one embodiment, the processor defines at least one of the boundaries (14, 22) based on a runway (12) position quality factor and an aircraft position quality factor. In another embodiment, the processor define the terrain clearance floor envelope (10) to be asymmetrical, such that respective profiles of the envelope (10) in a radial cross-section are different in at least two radial directions from a center point of the selected runway (12). The processor can define end regions of the terrain clearance floor envelope (10) adjacent to the ends of the selected runway (12) to have a different slope profile than side regions of the terrain clearance floor envelope (10) adjacent to the sides of the selected runway (12).