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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Image: Rocky Mountain National Park
The Geographic Names Information System (GNIS) was developed by the U.S.Geological Survey (USGS) in cooperation with the U.S. Board on Geographic Names (BGN), which maintains cooperative working relationships with state names authorities to standardize geographic names. GNIS contains information about the official names for places, features, and...
small excerpt of Yosemite Valley topographic map in shades of green and brown
They both might be correct. The reason that the elevations do not agree might be because they are set on different datums. Most maps use NAD27 and the elevations are based on mean sea level. Your GPS receiver uses WGS84 and the elevations are based on the NAD83 ellipsoid. These datum shifts can result in difference of tens of meters horizontal and...
Multi-temporal Landsat NDVI (Normalized Difference Vegetation Index) product overlaid on aerial photographs
The Spectral Characteristics Viewer is an interactive tool that can be used to visualize how the bands--or channels--of different satellite sensors measure the intensity of the many wavelengths (colors) of light. This is also known as the relative spectral response (RSR). By overlaying the spectral curves from different features (spectra), one can...
Moving Declination graphic
At most places on the Earth's surface, the compass doesn't point exactly toward geographic north. The deviation of the compass from true north is an angle called "declination" (or "magnetic declination"). It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time. It...
Thermal infrared imagery taken with drone
Color-infrared (CIR) aerial photography - often called false color photography because it renders the scene in other than the normal colors seen by the human eye - is widely used for interpretation of natural resources. Atmospheric haze does not interfere with the acquisition of the image, therefore is well suited to aerial photography. The red...
Photo taken from the air showing Hoover Dam with a lake backed up behind it and and a river flowing away from it.
USGS/EROS has created medium resolution digital scans at 400 dpi and high resolution scans at 1,000 dpi of the aerial photography products. Both the medium and high resolution aerial photography products can be downloaded at no cost via EarthExplorer or Glovis. A segment of the USGS archives remains on film media that has not yet been scanned....
Image: USGS Mapper at Work
The area covered by an air photo depends on the scale of the imagery. Most aerial photos were taken on 9 x 9 inch film. The chart below reflects coverage for a variety of photographic scales. Scale 1 inch = feet 1 side (miles) Area (square miles) 1:12,000 1000 1.70 2.9 1:20,000 1667 2.84 8.1 1:24,000 2000 3.41 11.6 1:40,000 3333 5.68 32.3 1:58,000...