The AHHI provides an indicator of the overall amount of U.S. EEZ that is currently surveyed to an appropriate level of certainty to support safe navigation. The index is measured based on an internal model that assessed the degree of certainty needed for a a given area based on several factors – for instance, highly tracked and shallower areas need higher resolution surveys, and must be resurveyed more frequently to capture natural and human-caused changes (e.g., from storms, dredging, or maritime accidents). The index has a maximum score of 300; areas that are more important to maritime industry are weighted more heavily in the index.