DCW Drainage Features, Points
- This datalayer is a point coverage representing hydrographic drainage features located throughout DCW (Digital Chart of the World) geographic coverage as of 1992. The minimum feature size for all polygon feature layers within the Digital Chart of the World is 0.12 inch (circumference measure). Drainage coverage features smaller than this size are retained as point locations in this layer. There are two exceptions to this rule: 1) lake features comprised of more than one line primitive (as when two different streams intersect a lake) are retained as drainage polygon features, and 2) glaciers smaller than the minimum size criteria have been eliminated from the database (without representation in the Drainage points coverage). This layer was derived from the VMAP0 series produced by the National Geospatial-Intelligence Agency (NGA), formerly known as NIMA (National Imagery and Mapping Agency), an agency of the United States government. VMAP data set products use the "vector product format" (vpf). The term "VMap" is derived from "Vector Smart Map." Digital Chart of the World is a comprehensive 1:1,000,000 scale vector basemap of the world. It consists of cartographic, attribute, and textual data. The primary source for the database is the National Imagery and Mapping Agency's (NIMA) Operational Navigation Chart (ONC) series. This is the largest scale unclassified map series in existence that provides consistent, continuous global coverage of essential basemap features. The coverage is is organized into 10 thematic layers which feature a vast number of files organized into libraries. These libraries, or coverages, are geographicially bounded areas, rather than feature-linked coverages. DCW Layers in the Harvard Geospatial Library are feature-based as we have processed the multiple tiles from the VMAP0 libraries into a series of seamless coverages (e.g. aeronautical features appear in a single layer, "Aeronautical Facilities, Points") which depict a particular feature as it occurs throughout the available geographic coverage areas. Since different geographic areas were mapped by NIMA at different times using various types of feature capture and scanning and digitizing methods and software, multiple map sources of varying dates and multiple processing approaches are represented in each of these HGL layers. We have necessarily generalized the discussion of data source material and process steps in the Data Quality section of this metadata document.
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