Most often, you will look for GIS data on the web using Google, but you will rarely find specific datasets. Instead, you will find repositories of data that you will then search for your specific needs.
When searching for data, choosing a repository at the right scale is a major issue. For example: if you are looking for country-by-country demographic information, choose a global repository, such as the United Nations or the World Bank.
Part of the OpenGeoPortal consortium, Stanford's EarthWorks allows you to search the digital data and scanned map libraries of several different institutions. Because this project is at Berkeley, it is especially useful when looking for California data. However, Tufts and Harvard are two partners, therefore you may get lots of results from Massachusetts when conducting searches. Make sure to limit your search to the extent that you want to compute on. While each OGP instance theoretically searches the entire corpus all of the instances, in practice there are differences. Other very useful nodes include:
Cal-Atlas GeoSpatial Clearinghouse (formerly CaSIL)
The official repository of several different California state agencies. Especially useful for statewide data. You wouldn't look here for data about Oregon.
The US federal government's main GIS repository. This is an ongoing effort: while the search engine is robust, you may still need to search at an individual agency when looking for more specific data. For example: while you can search for census data here, if you need to see a list of individual census datasets, you would be better off using American FactFinder.
Region 5 (Pacific Southwest) Geospatial Data
Our local Los Padres National Forest does not have its own spatial data web page, therefore you want to look at the regional level, where the Forest has 3 datasets available for download. Don't be fooled by the link in this sentence: "Other GIS data sets may be found on the Los Padre National Forest's Geospatial Data website." There's no data there.
A note about references to ADL:
The Alexandria Digital Library project conducted research throughout the 1990s on how to describe and provide access to spatially referenced information. The catalog produced by these efforts has largely fallen offline. The Library is strongly interested in reviving this research program with interested campus partners, and is currently creating back-end infrastructure in support of an eventual GIS data discovery and distribution tool.
If you are looking for GIS data that is outside the scope of the Library's collection, you may find it useful to refer to the resources below.
Should you decide to turn to Google in your search for GIS data, keep these tips in mind:
Source: Yale University Library
We define Southern California as the coastal region from San Luis Obispo County on the north to the Mexican border and includes the greater metropolitan areas of Los Angeles and San Diego and the inland counties (Kern, San Bernadino, Riverside, and Imperial)
Santa Barbara County
Our local county has removed its GIS information from its public web pages. The UCSB Library has several iterations of SBCo data dating back to the early 2000's.
Los Angeles County Enterprise GIS
Los Angeles County Data Portal
Southern California Association of Governments
Provides access to spatial data as well as an Interactive Atlas and a list of existing maps that are available from SCAG publications or have been created for other planning activities.
California GeoPortal
Includes the California Coastal GeoPortal, interactive map viewers, live map services, and links to many departmental data sites at the state level.
Cal-Atlas
Hierarchical lists of statewide data by theme from the California Resources Agency, facilitating data accesss to watershed and landuse planning.
Open Topography is based at the San Diego Supercomputer Center at University of California, San Diego. Open Topography provides access to high-resolution topography data acquired with lidar and other technologies.
A drop-down list of geospatial clearinghouses state-by-state from the MIT Libraries
Geospatial and attribute links for each U.S. state.
Data & Mapper Sources for U. S. States
http://researchguides.dartmouth.edu/content.php?pid=14880&sid=99903
GeoPlatform.gov
"Geospatial One-Stop" U.S. Maps and Data Hosts U.S. federal, state, and local geographic maps and data. The self-proclaimed "one stop for Federal, State, and Local Data", this portal provides search capability across multiple government bodies producing GIS data. You can search or browse by category, as well as save your searches and datasets.
ArcGIS Online
"Provides maps and data for ArcGIS users and the capability to find and share geographic information. The free ArcGIS Online Web application makes it easy to find content published by ESRI, ArcGIS users, and other authoritative data providers as well as upload and share content of your own."
NationalAtlas.gov
Use the "Map Maker" to create a map online with the map layers or download the raw data for your own analysis. Map layers include agriculture, biology, boundaries, climate, environment, geology, history, people, transportation and water. Not downloadable?
National Historical Geographic Information System (NHGIS)
Provides, free of charge, aggregate census data and GIS-compatible boundary files for the United States 1790 to 2000? Present?
Census TIGER Shapefiles
TIGER Line files for legal and statistical areas in shapefile format, updated annually. A collection of various Census shapefiles (boundaries and geographic features) in TIGER form.
US Census Cartographic Boundary Files
Generalized extracts of the bureau's TIGER Line files for several administrative, legal, and census areas in the US
National Land Cover Database
Land use and land cover data for the entire US
Libre Map Project
Topographic maps (DRGs) for the entire US, 2003 TIGER Line data, and SVG maps for several administrative areas
GeoFRED
Provides a map display for economic data, with download capability.
UDSA Geospatial Gateway
A collection of GIS data on natural resources.
The US Geological Survey (USGS) provides many GIS data resources:
UNEP Environmental Data Explorer
United Nations Environment Programme’s (UNEP) online database holds more than 500 different variables, as national, subregional, regional and global statistics or as geospatial data sets (maps), covering themes like Freshwater, Population, Forests, Emissions, Climate, Disasters, Health and GDP. Display them on-the-fly as maps, graphs, data tables or download the data in different formats. Click "Advanced Search," select "Geospatial Data Sets" under the 1st drop down box, and hit the red "Search" button for a list of global or continental GIS files
GeoNetwork
United Nations Spatial Data Infrastructure (UNSDI), Netherlands Coordinating Office
GeoNetwork (Food and Agriculture Organization of the United Nations)
Provides searchable and browsable access to International GIS data.
gData
Project at UC Berkeley that provides boundary files and imagery for every country in the world, tied to the DIVA GIS project
USGS Global Visualization Viewer
US federal agency that provides LANDSAT data (global satellite imagery) for the world
Center for International Earth Science Information Network (CIESIN)
Hosted by Columbia University. Includes links to datasets for the world, various countries, and the US
University of Arkansas Libraries
Provides excellent links for country-level GIS repositories.
ICSU World Data System Portal
Network of data repositories for worldwide GIS data. A project of the International Council for Science.