Maps and Mapping

The old coordinates were not wrong, just different. Positions obtained using the North American Datums of 1927 ( NAD 27 ) and 1983 ( NAD 83 ) are based on different earth shapes--or ellipsoids--and used the best technology available at the time. Mathematically, NAD 83 is a stronger datum because all previously existing horizontal stations and newer GPS surveyed stations were adjusted...

The National Geologic Map Database (NGMDB) is an archive of geoscience maps (including geology maps), reports, and stratigraphic information for the United States. The NGMDB contains information on more than 90,000 maps and related geoscience reports published from the early 1800s to the present day, by more than 630 agencies, universities, associations, and private companies. Much of the content...

Original hydrographic data were compiled to meet National Map Accuracy standards . These standards have been maintained in the process of creating the NHD. At 1:100,000 scale, ninety percent of well-defined features are within 167 feet (0.02 inches at map scale) of their true geographic position. When higher resolution data are created and submitted, the positional accuracy may vary due to...

Boundaries, structures, and transportation data products are currently available in Esri File Geodatabase 10.1 or Shapefile format. Use The National Map Downloader , The National Map Services , or download files staged at the below sites. USGS National Boundary Dataset USGS National Structures Dataset USGS National Transportation Dataset

A Geographic Information System (GIS) is a computer system that analyzes and displays geographically referenced information. It uses data that is attached to a unique location. Most of the information we have about our world contains a location reference: Where are USGS streamgages located? Where was a rock sample collected? Exactly where are all of a city's fire hydrants? If, for example, a rare...

A digital orthophoto quadrangle (DOQ)--or any orthoimage--is a computer-generated image of an aerial photograph in which displacements (distortions) caused by terrain relief and camera tilts have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map. Unlike an aerial photograph, an orthoimage has a uniform scale, so it can be used as a base map...

UTM is the acronym for Universal Transverse Mercator, a plane coordinate grid system named for the map projection on which it is based (Transverse Mercator). The UTM system consists of 60 zones, each 6-degrees of longitude in width. The zones are numbered 1-60, beginning at 180-degrees longitude and increasing to the east. The military uses their own implementation of the UTM system, called the...

The National Hydrography Dataset (NHD) is a digital vector dataset used by Geographic Information Systems (GIS) to define the spatial locations of surface waters and is designed to provide comprehensive coverage of surface water data for the U.S. The NHD contains features such as lakes, ponds, streams, rivers, canals, dams, and stream gages in a relational database model system. These data are...

A Digital Elevation Model (DEM) is a representation of the bare ground (bare earth) topographic surface of the Earth excluding trees, buildings, and any other surface objects.  DEMs are created from a variety of sources. USGS DEMs used to be derived primarily from topographic maps . Those are being systematically replaced with DEMs derived from high-resolution lidar and IfSAR (Alaska only) data...

Scales for data in the National Hydrography Dataset: Conterminous United States, Hawaii, and the Virgin Islands, medium resolution - 1:100,000-scale Conterminous United States, Hawaii, and the Virgin Islands, high resolution - 1:24,000-scale Puerto Rico - 1:20,000-scale Alaska - 1:63,360-scale Users are encouraged to work with their state's principal steward to produce and submit data at higher...

Within the conterminous 48 states, the North American Datum of 1927 (NAD 27) to the North American Datum of 1983 (NAD 83) shift of the latitude/longitude graticule (lines showing parallels of latitude and meridians of longitude for the earth) is in the range of 10-100 ground meters. Changes to UTM values are generally larger, around 200 meters, and changes for other coordinate systems are...

The distances vary. A degree, minute, or second of latitude remains fairly constant from the equator to the poles; however a degree, minute, or second of longitude can vary greatly as one approaches the poles and the meridians converge. At 38 degrees North latitude (which passes through Stockton California and Charlottesville Virginia): One degree of latitude equals approximately 364,000 feet (69...