Frequently Asked Questions
Mapping, Remote Sensing, and Geospatial Data
The USGS provides the mapping and digital geospatial foundation for the Nation.
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• US Topo
3DEP standard DEMs are produced from the highest quality elevation sources within the data holdings of the USGS National Geospatial Program. As of 2015, about 25% of the conterminous U.S. coverage is from high resolution lidar and photogrammetric source, while the remainder of the conterminous U.S. and Hawaii coverage is from USGS topographic map...
The projection used for all tiled base map services is the WGS84 Web Mercator (Auxiliary Sphere) so base map services can be used in combination with other common viewers, such as Google Maps and Microsoft Bing Maps. For more info: see Spatial Reference SR-ORG:6928 and Spatial Reference SR-ORG:7483/.
Dynamic overlay services are in WGS84 (see ...
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.
File sizes to store tiles increase 4x with each zoom scale when building a cache for a tiled base map service. The storage size required to cache data at scales larger than 1:18,000 is greater than USGS can currently store. For example, just to store data down to 1:18,000 requires a cache of over 39 million tiles. Thus, high-performing dynamic...
A tiled service contains cached tiles, which render the map in small tile images. This allows the map to perform much faster when panning and zooming, compared to a dynamic service. Dynamic is considered an "older" way of displaying web maps, where each feature is called from the source data and drawn separately for every pan and zoom. Tiled base...
The National Map offers a series of composite base maps, which 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...
No, but additional features in The National Map are found in dynamic services. Expand the Overlays pane above the Content sub-tab, expand individual Base Data Layers by clicking on the + signs, and select which additional features you may want to view at various scales. Symbol legends can be viewed by opening additional content below each ...
Vector data for small scales are from The National Map Small-Scale Collection (formerly National Atlas), while medium to large scales are comprised of The National Map themes, to include the National Hydrography Dataset (NHD) and Geographic Names Information System (GNIS) data. Other medium to large scale vector features are from USGS edited...
USGS only offers vector downloads in bulk by special request. For assistance, send an e-mail to tnm_help@usgs.gov and include your contact information, requested product, requested area, and download format.
The most recently published figure of absolute vertical accuracy of the 3DEP DEMs within the conterminous United States, in terms of the National Standard for Spatial Data Accuracy (NSSDA) at 95% confidence level, is 3.04 meters.
It is important to note that the vertical accuracy actually varies significantly across the U.S. because of...
The area of coverage of any particular standard 3DEP DEM product is generally determined by the availability of source elevation data of similar or higher resolution. For example, the 1/3 arc-second layer is available for only portions of Alaska where higher resolution lidar or ifsar elevation sources exist. The 1/9 arc-second layer is available...
All 3DEP seamless DEMs are provided in geographic coordinates (longitude and latitude) in units of decimal degrees, horizontally referenced on the North American Datum of 1983 (NAD83).
All elevation values are in units of meters, typically referenced to the North American Vertical Datum of 1988 (NAVD88), although the National Geodetic Vertical...