PO area features were automated using both scanning/vectorization and manual digitizing techniques. PO lines comprising area features were extracted from two primary types of source film separates: 1) drainage/coastline (including single line rivers that form political boundaries), and 2) culture (for explicit depiction of political boundaries on source). Additional information on techniques for automating political boundaries is presented in the narrative file to the POLINE feature class.
Feature coincidence
All shared boundaries between PO area features, inland water bodies, city outlines, and land cover area features have coordinate coincident representations. Single line rivers which terminate at coastlines intersect these features at single coordinate coincident points. In some instances, single line rivers were extracted from the DNLINE feature class to fill gaps in international boundaries as depicted on source manuscripts. Artificial closure lines at the boundaries between ocean aggregations and inland water bodies have identical representations in the PO and DN coverages.
Database design issues
Political boundaries were not always well defined on source manuscripts. This may have significant local effects on country code assignments to island features in the vicinity of international boundaries Ambiguous cases were resolved using DIAM 65-18 (March 15th, 1984) as a guide. Cases unresolvable using the DIAM standard were coded as unknown. Area features with a circumference less than 0.12 inch were collapsed to point features and placed in the POPOINT feature class. Updates to country status codes was frozen on 1 November, 1991. Countries established after this are not present in the database.