ASRIS Technical Specifications V1.6

8 Level-3 descriptors (land zone)The land units at Level 3 are predominantly groupings of geomorphically related systems. Some may alsoreflect administrative boundaries used by the states. These units differ from Level 2 in both detail andconcept. Level-3 units may have many tracts and conformity of boundaries with Level 2 is not mandatory.Level-3 tracts are normally aggregated from surveys and are below the ASRIS Mapping Hiatus whereasLevels 1–2 are based on subdivisions of the continent. Most descriptors for Level 3 are generated assimplifications, weighted averages and histograms from Level-4 and Level-5 tracts. Additional descriptionsprovide:• Estimates of the resistance to weathering of the substrate• A text description of the tractIn many cases, the number of Level-3 tracts per Level-2 parent will be greater than seven.9 Level-4 descriptors (land district)The land units at Level 4 are normally groupings of geomorphically related systems. (In some states, landsystem or soil-landscape systems are used for these units.) Where these are not the original scale of survey,Level-4 tracts are normally aggregated from surveys as they are below the ASRIS Mapping Hiatus. In theintensive land-use zone most descriptors for Level 4 are generated as weighted averages and histogramsfrom Level-5 tracts. Additional descriptions provide:• Estimates of the resistance to weathering of the substrate• A text description of the tractIn many cases, the number of Level-4 tracts per Level-3 parent will be greater than seven.24