Thematic map data is typically the most difficult data type of the three (thematic, topographic, remote sensing) to find in digital format. In large part this is because the demand for digitization of analog thematic maps to digital formats lags behind the demand for the transition of other data types. Nonetheless, availability of high-quality digital thematic data is increasing yearly, as is the availability of tools to find such data. Open access to thematic data is greatest in the United States, and so here we will focus on locating such data in the U.S. For links to resources on geospatial data, geologic data, and geophysical data, visit the MIT Center for Digital Geology (MCDIG) data sources webpage.
National Data Resources
Most thematic data are available on a local (state or county) level; however there are a few resources that provided nation-wide data, or the ability to search for data on a national scope.
National Geologic Map Database
The National Geologic Map Database is an excellent resource for finding published geologic maps across the United States. The entire map catalog of United States Geological Survey is searchable through the database, as are most geologic maps published by state geological surveys. Many maps published by societies (e.g. Geological Society of America) are searchable in the database, as are many thesis maps.
Searches in the National Geologic Map Database can be done by latitude/longitude, place name, quadrangle name, state, and county. Options to specify particular types of maps (bedrock, surficial, geochemical) are available, as are options to limit search results to a particular scale (e.g. greater than 1:100,000) or to a particular format (digital only). A Google Earth compatible search of the National Geologic Map Database is available on a trial basis for several western states.
Digital versions of USGS geologic maps can be directly downloaded from the National Geologic Map Database. Older paper maps are frequently available as MrSID format images, some of which are georeferenced. The National Geologic Map Database provides a help page for working with MrSID map images, including a georeferencing tutorial for users of ArcGIS to illustrate how to georeference these map images.
Newer USGS digital geologic maps are often available as ArcGIS coverages, shapefiles, or databases which can be directly imported into GIS software.
For digital maps published by state geological surveys, the database often contains a hyperlink to a location where the digital map can be obtained. Frequently these data are hosted at a state geological survey website.
The National Atlas
An excellent source for thematic data are the map layers available through the National Atlas. The National Atlas contains a broad cross section of nationally relevant data in subjects such as:
- Agricultural Data including crops, expenses, farmland
- Biological Data including land cover, forests, invasive species, animal distributions
- Public Land and Political Boundaries including Congressional districts, counties, state and federal lands
- Climate Data including precipitation data, hurricanes, sea temperature
- Environmental Data including air releases, toxic waste, hazardous materials
- Geologic Data including earthquakes, landslides, volcanoes, shaded relief
- Map Reference Data including cities and towns, urban areas, public lands
- Population Data including census, crimes, mortality
- Transportation Data including roads, railroads, and airports
- Water including streams, dams, aquifers, lakes, watersheds
United States Department of Agriculture Geospatial Gateway
The USDA Geospatial Gateway is an excellent resource for thematic data as well as topographic and remote sensing data. This Geospatial Gateway contains digital soil surveys for most of the United States, including county soil maps, soil profiles and soil reports, as well as nation-wide precipitation and temperature data. Soil maps and reports are also available on a state-by-state basis through many state geospatial data services.
State and Local Thematic Data Resources
The number of state and county thematic data resources available throughout the United States is simply to great to comprehensively list, and the web links, particularly to smaller agencies, tend to change fairly frequently. Some good starting points are listed below, although an internet search on the state or county that you are interested in, along with the data type, will often yield fruitful results. Geologic data are often hosted at state geological surveys, departments of mines and geology, or departments of natural resources.
State Geological Survey Maps
Many thematic maps at a large scale (e.g. 1:24,000) are distributed through various state geological surveys. The format of distribution varies widely from state to state and may include georeferenced images such as GeoTIFFs or MrSID images, GIS databases, or, more commonly, PDF files. PDF files typically need to be converted to a JPEG or TIFF format before importation into a GIS software system. A number of image processing software packages will undertake this conversion. The resulting image file would need to be georeferenced.
Many of the maps produced by state geological surveys are funded as part of the State Geologic Survey Mapping Component. This web site contains information about geological mapping in each state, although most of these maps are also searchable in the National Geologic Map Database. A few examples are listed below, although similar resources exist for many other states.
- Geologic maps at the New Mexico Bureau of Geology and Mineral Resources
- Nevada Bureau of Mines and Geology On-line Documents, including geologic maps
- Montana Bureau of Mines and Geology geologic quadrangles