The original GAP datasets did not align along state boundaries, and additionally, the polygons were not edgematched. To join these datasets, all coverages were projected to the current Albers projection. Next, a coverage for each state was created so that each contained only the item LEVEL3CODE, the finest level of the coding system. The LEVEL3CODE was determined by incorporating all of the available ownership and management attributes for each state. Polygon topology was verified and corrected as necessary. The coverages were then appended and topology rebuilt with the ArcInfo Clean command. Newly created polygons along state boundaries with a LEVEL3CODE value of 0 were then Eliminated since they were artifacts of the state boundaries misalignment. The coverage was then dissolved on PROTSTAT to delete the boundaries between like attributed polygons across state borders. The additional attributes (LEVEL1CODE, LEVEL1NAME, LEVEL2CODE, LEVEL2NAME, and LEVEL3NAME) were subsequently joined from a look-up-table and the coverage was clipped to the US Columbia River Basin boundary.
Please note that due to time and funding constraints, the newly added (non-GAP) codes do not perfectly nest hierarchically. The codes were added as each state was processed without changing any of the GAP stated codes. These data could be improved by creating a new hierarchical system (it would have to alter the structure of the GAP system slightly) and recoding the data. Additionally, if a new coding system were adopted, the original data should be re-evaluated one more time to ensure classification accuracy as some of the earlier classified states may have polygons that could be more accurately classified by classes that were added later in NHI's coding process.