This datalayer is a polyline coverage representing bridges, overpasses or viaducts located throughout current Vmap geographic coverage areas.
Objects in this layer have FACC (Feature and Attribute Coding Catalogue) Codes of AQ040, defined as, "Bridge: A man-made structure spanning and providing passage over a body of water, depression, or other obstacles." Additonal attributes describe the type of structure or mechanism by which the bridge might be moved to allow for the passge of vessels; the structural design characteristics of the bridge or bridge segment (floating, suspension, cantilever, etc.); the primary use of the bridge (road, railroad, pipeline, etc.); bridge height and length and more.
This layer was derived from the VMAP1 series produced by the National Geospatial-Intelligence Agency (NGA), an agency of the United States government. VMAP data set products use the "vector product format" (vpf) utilized first by the US Defense Mapping Agency and later by its successor, the National Imagery and Mapping Agency (NIMA, now known as the NGA). "VMap" is derived from "Vector Smart Map"
Vector Smart Map (VMAP) Level 1 is a global vector GIS dataset at 1:250,000 scale. This is currently the most detailed global digital dataset freely available and can be downloaded from the National Imagery and Mapping Agency (NGA) website, using the GeoSpatial Engine The coverage is not complete - of the 234 zones which make up the dataset, only around 25% are currently (2002) available - others have not yet been completed or are not declassified. They feature a vast number of files organized into libraries. These libraries, or coverages, are geographicially bounded areas, rather than feature-linked coverages.
VMAP Layers in the Harvard Geospatial Library are feature-based as we have processed the multiple tiles from the VMAP libraries into a series of seamless coverages (i.e. Aqueduct Lines appear in a single layer, "Aqueduct Lines") 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.