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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Thumbnail of a topographic map with a grid of individual indexes and their names.
View map indices on these interactive maps (you must zoom in to see the index lines and map names): Map Locator on the USGS Store - Click the icon on the left that looks like two pieces of paper and select “1:24,000”. TopoView - Click on any of the round map scale indicators on the right side of the map.  The National Map Viewer - Click on the...
Image: Cartographers in the Field
The distinctive characteristic of a topographic map is the use of elevation contour lines to show the shape of the Earth's surface. Elevation contours are imaginary lines connecting points having the same elevation on the surface of the land above or below a reference surface, which is usually mean sea level. Contours make it possible to show the...
Image: Alaskan Elders
Once you've gathered as many facts about family history and customs as possible, turn to maps to uncover more specific information or to solve historical mysteries. In the United States, birth, death, property, and some other kinds of records are normally kept by county governments. If you can name the place where an ancestor lived, new or old...
Vertical line with a star at the tip, second angled line with GN at the tip, third angled line with MN at the tip.
A diagram at the bottom of most USGS topographic maps shows three north arrows--true north, grid north, and magnetic north--and the angles between them. Some maps, especially very old maps, do not have this diagram. Example of north arrows from US Topo map. Star is True north, GN is Grid north, MN is Magnetic north. (Public domain.)   True...
1914 USGS legacy topographic map of the Muscle Shoals, Alabama
A USGS topographic map is usually named for the most prominent feature within the bounds of the map, which is frequently a community. Most topographic maps are named for the most centrally located, well-known, and/or largest community identified on the map. If the community for which the map should be named falls on two or more maps, a directional...
Crop of Landsat 7 Path 38 Row 35, Acquired March 29, 2013
Landsat 7 ETM+ SLC-off data refers to all Landsat 7 images collected after May 31, 2003, when the Scan Line Corrector (SLC) failed. These products have data gaps, but are still useful and maintain the same radiometric and geometric corrections as data collected prior to the SLC failure. See the Landsat 7 website for information about the SLC-off...
Landsat image showing scar marks from fire in comparison to ASTER.
Data acquisition requests are not available to the public. The Landsat 8 and Landsat 7 satellites each acquire data on a continual basis in accordance with their respective Long Term Acquisitions Plans. The Landsat Acquisitions page has calendars and information about daily schedules. Learn more: Landsat 8 Long Term Acquisitions Plan Landsat 7...
Image shows aerial imagery of flooding in Louisiana, courtesy of the National Geodetic Survey
Remote sensing is the process of detecting and monitoring the physical characteristics of an area by measuring its reflected and emitted radiation at a distance (typically from satellite or aircraft). Special cameras collect remotely sensed images, which help researchers "sense" things about the Earth. Some examples are: Cameras on satellites and...
Lidar image of Pittsburgh
Lidar data are available through The National Map Download Client. Click the “How To” link at the top of the viewer for detailed instructions on how to find and download data.   Light Detection and Ranging (LIDAR) is a technology similar to RADAR that can be used to create high-resolution digital elevation models (DEMs) with vertical accuracy as...
Aerial photo of Anvil Lake
Yes, there is a plan to scan and digitize all rolls of film. The plan is to first scan all rolls of film that are in danger of decay from vinegar syndrome and then scan everything else. Science and customer requirements are also reviewed on a regular basis, but a formal, on-line schedule is not available.
High Resolution Orthoimagery coverage map on The National Map
The U.S. Geological Survey (USGS) Earth Resources Observation and Science Center (EROS) can only certify that the product downloaded through our website is an authentic reproduction of an official record of the Government that is in the legal custody of the USGS EROS.
Orthoimagery of the Maryland State House, Annapolis, Maryland
It will depend on the resolution and scale of the imagery. High Resolution Orthoimagery and National Agriculture Imagery Program (NAIP) imagery both have a resolution of 1 meter or less so you should be able to see objects as large as houses.  What you can easily see in an aerial image is mostly a function of scale and resolution. Film resolution...