There are many sources for geospatial information and data. The US federal government has created a portal known as the Geo.Data.gov (http://geo.data.gov/), part of Data.gov, to promote communication and sharing of geographic data by all levels of government and the public. GIS data are also available at the state and local government levels, from commercial vendors, and from other organizations.

Some sources of GIS data include:

United States Federal Government

• Geo.Data.gov - http://geo.data.gov/
• US Census Bureau - http://www.census.gov/geo/www/index.html
• US Census Bureau State & County QuickFacts- http://quickfacts.census.gov/qfd/index.html
• National Atlas of the United States - http://www.nationalatlas.gov/
• National Aeronautics and Space Administration (NASA) - http://www.nasa.gov/home/index.html
• EPA National Geospatial Program - http://www.epa.gov/geospatial/
• Federal Emergency Management Agency (FEMA) - http://gis.fema.gov/
• National Geodetic Survey - http://www.ngs.noaa.gov/
• National Cancer Institute - http://seer.cancer.gov/

Other

• The Geography Network - http://www.geographynetwork.com/
• GIS.com - http://www.gis.com/ (information and links)
• Center for Agricultural, Resource and Environmental Systems (CARES) (http://www.cares.missouri.edu/), University of Missouri at Columbia

Simply defined, metadata is data that describes other data. It provides additional information about the data that the GIS uses, which is not included in the data itself, such as date of origination, currency, accuracy, extent, format, custodianship, and collection methodology. Metadata is typically stored in data models, dictionaries, schemas and other representations. Spatial metadata supports easier data access and management. The adoption of metadata standards promotes consistency in the naming, defining, operating, and cataloging of data among different entities.

To display an accurate visual representation of a location, a GIS uses an absolute location reference system. Latitude and longitude, written in degrees (°), is one such system.

Latitude lines, known as parallels, run horizontally, and measure the distance north or south of the Equator. The Equator is the starting point of measuring latitude and is 0° latitude. Latitude is measured in degrees from 0° to 90° and is specified as North or South.

Longitude lines run vertical and are known as meridians. They indicate the distance east or west of the "prime meridian." The prime meridian runs through Greenwich, England and is 0°longitude.

Many US federal government organizations, as well as state and local governments, use GIS to display health-related information. Some US federal sites include:

• Cancer.gov (http://gis.cancer.gov/) from the National Cancer Institute (NCI) (http://www.cancer.gov/) is a central source of information about GIS and related resources for use by the public, cancer researchers, and the GIS Special Interest Group (http://gis.cancer.gov/overview/gissig.html).
  
  
• EnviroMapper StoreFront (http://www.epa.gov/enviro/html/em/) from the EPA (http://www.epa.gov/) helps you view federal, state, and local information about environmental conditions and features in an area of your choice.
  
  
• MyEnvironment (http://www.epa.gov/myenvironment/) is a powerful web-based tool from the EPA (http://www.epa.gov/) that provides a wide range of federal, state, and local information about environmental conditions and features in an area of your choice.
  
  
• GATHER, Geographic Analysis Tool for Health and Environmental Research, (http://gis.cdc.gov/) from the CDC (http://www.cdc.gov/) is an online spatial data access system that provides members of the public health community and general public access to spatial data that is pertinent to the analysis and exploration of public health issues.