Frequently Asked Questions
Mapping, Remote Sensing, and Geospatial Data
The USGS provides the mapping and digital geospatial foundation for the Nation.
Browse Mapping, Remote Sensing, and Geospatial Data science related to:
• US Topo
The Landsat Program is a series of Earth-observing satellite missions jointly managed by NASA and the U.S. Geological Survey.
On July 23, 1972, in cooperation with NASA, the Earth Resources Technology Satellite (ERTS-1) was launched. It was later renamed Landsat 1. Additional Landsat satellites followed in the 1970s and 1980s. Landsat 7 was...
Yes, depression contours are identified with tick marks, but only in large scale contours from 36K to 18K.
The 500-foot contours are shown at a scale of 578K in base maps, while 100-foot contours are visible at 289K/144K and 50-foot contours are visible at 72K. Large scale contours from US Topo products are shown from 36K to 18K, and in dynamic base map services for scales larger than 18K.
Contours cover the conterminous United States at small (578K/289K), medium (144K/72K), and large (36K/18K) scales.
When significant changes in the landscape have occurred, contours will be updated on an as-needed basis.
The 100-foot contours were derived from 3DEP (formerly National Elevation Dataset) one arc-second resolution data, which was sub-sampled to a cell size of three arc-second. The 50-foot contours were also derived from one arc-second data. Large...
Contours are not projected, but are provided in geographic coordinates (latitude and longitude) in units of decimal degrees, and horizontally referenced to the North American Datum of 1983. Contour elevation values are vertically referenced to the North American Vertical Datum of 1988. Contours are staged in Esri File Geodatabase 10.1 or Shapefile...
Updated digital elevation model (DEM) datasets are now renamed with a different naming convention to include resolution and format in the file name. This prevents users from accidentally overwriting existing data with the same file names across multiple resolutions and formats.
For example, an older dataset named n44w071.zip has been replaced with...
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 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...
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.
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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...
A tiled service contains cached tiles that 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...