Vector data for small scales are from The National Map Small-Scale Collection, while medium to large scales are comprised of The National Map themes, including the National Hydrography Dataset (NHD) and Geographic Names Information System (GNIS) data. Other medium-to-large-scale vector features are from USGS edited versions of the U.S. Census Bureau's TIGER/Line data for Governmental Unit Boundaries, Structures, and Transportation.
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What map projections are used in The National Map tiled base map services and dynamic overlay services?
The projection used for all tiled base map services in The National Map is the World Geodetic System 1984 (WGS 84) Web Mercator (Auxiliary Sphere) so that base map services can be used in combination with other common viewers. See Spatial Reference SR-ORG:6928 and Spatial Reference SR-ORG:7483. Dynamic overlay services are in WGS84 (see Spatial Reference EPSG:4326) but can be reprojected as needed...
How often are The National Map's composite base maps and dynamic service overlays updated?
Composite base maps are recached about once a year. Although data in dynamic service overlays are ideally updated quarterly, it can sometimes take up to six months. Please note that there will be times when composite base maps and dynamic service overlays are not in synchronization because of a difference in refresh cycles. Subscribe to the National Map Simple Notifcation Service (SNS) to stay in...
Why does The National Map offer dynamic base map services in addition to tiled services?
When building a cache for a tiled base map service, file sizes for tiles increase four times with each zoom scale. The storage size required to cache data at scales larger than 1:18,000 is greater than the USGS can currently manage. For example, just to store data down to 1:18,000 requires a cache of over 39 million tiles. High-performing dynamic services were created to support scales of 1:9,000...
What are the base map services (or URLs) used in The National Map?
The National Map Services offers a series of composite base maps that combine layers into tile cache services with the cartographic richness of multiple data sources. The following base maps are available:USGS Topo - combines the most current data (boundaries, geographic names, transportation, elevation, hydrography, land cover, and other themes) that make up The National Map. Contours generated...
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What map projections are used in The National Map tiled base map services and dynamic overlay services?
The projection used for all tiled base map services in The National Map is the World Geodetic System 1984 (WGS 84) Web Mercator (Auxiliary Sphere) so that base map services can be used in combination with other common viewers. See Spatial Reference SR-ORG:6928 and Spatial Reference SR-ORG:7483. Dynamic overlay services are in WGS84 (see Spatial Reference EPSG:4326) but can be reprojected as needed...
How often are The National Map's composite base maps and dynamic service overlays updated?
Composite base maps are recached about once a year. Although data in dynamic service overlays are ideally updated quarterly, it can sometimes take up to six months. Please note that there will be times when composite base maps and dynamic service overlays are not in synchronization because of a difference in refresh cycles. Subscribe to the National Map Simple Notifcation Service (SNS) to stay in...
Why does The National Map offer dynamic base map services in addition to tiled services?
When building a cache for a tiled base map service, file sizes for tiles increase four times with each zoom scale. The storage size required to cache data at scales larger than 1:18,000 is greater than the USGS can currently manage. For example, just to store data down to 1:18,000 requires a cache of over 39 million tiles. High-performing dynamic services were created to support scales of 1:9,000...
What are the base map services (or URLs) used in The National Map?
The National Map Services offers a series of composite base maps that combine layers into tile cache services with the cartographic richness of multiple data sources. The following base maps are available:USGS Topo - combines the most current data (boundaries, geographic names, transportation, elevation, hydrography, land cover, and other themes) that make up The National Map. Contours generated...
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