|Data Models||Data Applications||Data Formats||Data Sources|
The following is an attempt to provide professionals and students alike with a "one-stop-shopping" point of reference for public access data. Because of time constraints on this project, I focused this effort on data sources relative to Nebraska. Working within this limitation, I feel I have brought together some very useful data sources for geospatial research projects.
Before pressing on and discussing sources of data, I feel it is appropriate to briefly mention the nature of the two most prevalent data structures or models in use: raster and vector. Additionally, I will look at a few specific applications each model is best suited to.
Raster data. The raster-based data model is made up of rows and columns whose intersections define cells, very much like a spreadsheet. These cells are also referred to as pixels (from picture elements) when dealing with images. Because there are no "coordinates" (x-y pairs, latitude-longitudes, easting-northings, etc.), map-like linear features are not present. Instead, the cells are assigned values representing conditions of an area. The size of that area represented by a cell determines the spatial resolution of the database. An example might be inputting surface air temperatures into the database for a county. Each cell of the raster model could represent 1 square mile (or a larger or smaller area depending on the resolution sought). If these temperatures are systematically input so that adjacent areas on the Earth correspond with the model, then a raster model will have been created.
Vector data. Vector-based models are more representative of the world we live in. Vector data are generated from a coordinate system. Points (an x-y pair) are the basic element to a vector model. Lines, or arcs, are constructed by connecting points. Polygons, which define areas, are constructed of interconnected arcs. Examples would include road maps, maps of administrative boundaries, or a map of streams (see image below right). Rights-of-way, borders, and stream channels would be represented by lines, road intersections by points, and features along a road or lakes as polygons, as well as points and lines depending on scale.
Fortunately, or unfortunately, these somewhat tidy definitions get cloudy when the concept of attribute data is introduced. In order to gain meaningful information from these models, additional data are required. Attributes are data attached to the spatially located points, lines, areas, and cells. For the vector model, attributes typically describe one element's relationship to another. For raster data, a cell may contain an index value which refers to an attribute file describing that place on the ground in several ways.
A whole chapter, if not a book, could be devoted to the discussion of these data models. This is beyond the scope of this document. Several sources of information exist for further study of these models. Below are a couple of links to textbooks addressing the subject for further reading.
Getting Started with Geographic Information Systems by Keith C. Clarke
to Maps and GIS or Maps and GIS: The Internet Approach
The raster model, due to its construction, portrays fuzziness well. What does this mean? The natural Earth has very few crisply defined boundaries and features. There usually is some sort of gray area or transition zone at boundary locations. Because of this, a raster model can represent natural features effectively. Its gridded structure prevents it from containing sharply defined features. A way to observe this fuzziness is to highly magnify a digital image. At these high magnifications the edges of objects in the image become ragged, taking on a stair-step appearance. This is due to the gridded make-up and cell sizes of the raster model.
Digital elevation models and remotely sensed imagery (like the Advanced Very High Resolution Radiometer image at left) are represented very well by the raster model. Analytical functions like map overlays, terrain analysis, and trafficability studies are conducted effectively using the raster model.
On the other hand, connectivity functions, point in polygon (used when determining if a point feature is located within an area feature), and line in polygon (used when determining what areas are intersected by line features) functions are adeptly handled by vector models.
This being the case, then it makes sense that a vector model is especially well suited to displaying cultural features. Utility companies and transportation departments utilize vector-based GISs to represent their linear subject matter: roads, rights-of-way, intersections, etcetera. Also, it is fairly easy to create buffer zones around vector data when conducting research about areas effected by highways, gas lines, or high-tension powerlines.
Raster and vector model data are used in several formats. Raster, the more widely used, has a couple of fairly well known formats. Because the raster model is suited to images, most of the formats for raster data are associated with pictures. Formats like the Graphics Interchange Format (GIF), Tagged Interchange Format (TIF), and Joint Photographic Experts Group (JPEG) are very common, with the first and latter especially so on the Internet. All of these formats use some form of data compression to reduce file size for storage and transmission. Additionally, these formats are best suited to certain circumstances. Briefly stated, the GIF format is 8-bit displaying 256 shades of gray or color and represents alpha-numeric symbols and line features well. The JPEG format is 24-bit and can handle over 16,000,000 colors, but because of its compression algorithm is best suited to natural subjects with minimal straight lines and text.
A final common and widely used raster format is the Digital Elevation Model (DEM) of the U.S. Geologic Survey (USGS). The image to the right is a DEM of Salt Lake City West, Utah. This format is more complex than the others mentioned above, because it has to be able to deal with variable map projections. Also, because DEMs are used in mapping and surveying applications, the format needs to be robust and insure a high degree of accuracy.
Among vector data formats, the Digital Exchange Format (DXF) developed by Autodesk, the Digital Line Graph (DLG) from the USGS, and the Topographically Integrated Geographic Encoding and Referencing (TIGER) files from the U.S. Census Bureau are commonly used. Another format widely used and accepted is the ARC/INFO format from the Environmental Systems Research Institute, Inc. (ESRI). This format is used by the company's ARC/INFO Geographic Information System software. This format and software program have approached becoming the industry standard for vector based data formats and geographic information systems. The major difference in these vector formats is the DXF format does not presently support topology; the describing of adjacent features and connectivity. The DLG, TIGER, and ARC/INFO data formats do, and as such represent true geographic information systems.
As can be seen from the previous paragraphs, sources for geographic data abound. All the sources mentioned above provide information on areas throughout the United States, as well as some foreign countries. For information specifically about Nebraska, and problems Nebraskan, the following sauces should be researched.
University of Nebraska
Within the University of Nebraska system, several sites exist providing a variety of geographic research data. A first place to check would be the Center For Advanced Land Management Information Technologies (CALMIT), at the University of Nebraska-Lincoln. This homepage is a good source of information, but for a GIS project requiring data the link to the digital databases available through the Conservation and Survey Division and CALMIT is a good starting place. This site provides descriptions of data bases for geographic information systems. Additionally, the data bases are broke out into vector and raster-based products. Most descriptions include an example of each product described.
The University of Nebraska-Institute of Agriculture and Natural Resources is not a site for data downloads via the Internet. However, the "Departments -- Centers -- Units" link on their homepage can put you in touch with over 30 associated sections possibly working the same, or similar projects as you are. An e-mail capability is provided on most links.
Finally, while presently not having readily accessible data files for download, the Environmental Monitoring and Resource Analysis Lab (EMRA) does display information regarding current research projects. By contacting the EMRA lab, a data sharing agreement may be possible.
Nebraska State Government
In addition to the University as a source of data, numerous state commissions provide geographic information. To start with there is the Nebraska Geospatial Data Clearinghouse. This is a searchable catalog of information about data covering areas of Nebraska. The clearinghouse contains no data per se, but consists of metadata (the data about data), and so is a good research site to determine the usability of data for a given project. State, local, federal agencies and businesses are contributors to this site.
The Nebraska Game and Parks Commission GIS Inventory is a relatively new program. The staff is using ESRI's ARC/INFO and ARC/VIEW software to generate various maps. Presently these maps are displayed in the GIF format. No option to download the data used for the maps was noted, however contacting the staff to see if this option is possible through another means is suggested. Maps are presented in three categories. The first is maps created with ARC/INFO. These maps include waterfowl zone maps, a Cowboy Trail Map, Central Platte River Habitat Tracts, and Platte River Properties. Maps created using ARC/VIEW include Commission Districts 5 and 7 property maps and a Whooping Crane Sighting Map. Finally, the third category consists of TIGER maps of Nebraska. Selecting this option will provide the visitor with maps of several Nebraska areas for viewing.
A final comprehensive source of data is the Nebraska Natural Resources Commission Data Bank, and specifically their SPATIAL/GIS DATABASES page. From this page you not only have access to many digital orthophoto quarter quadrangles (DOQs) and DEMs, but several other data bases. These data bases include, but are not restricted to, the following:
- Groundwater Vulnerability to Contamination, available in what is known as the ARC Export Format characterized by the .e00 file extension. Data files in this format are easily imported into ARC/INFO and ArcView.
- National Wetlands Inventory, available in html, DLG, and ARC Export Formats.
- State Soil Geographic Database, available in html and ARC Export Formats.
- Registered Ground Water Wells Database. This data base allows the researcher to query against registered wells in numerous ways to include by county, owner's name, and Natural Resource District. Metadata for this data base is available in html format. Data can be acquired in ARC Export Format.
A final note on the Natural Resources Commission is that if you should require data in a format not listed on their web page, you can submit a request to them asking for the format you need. It doesn't guarantee your request will be fulfilled, but is nice to know they will at least attempt to meet your needs.
- National Sources
Should a search of the above state sources not provide the data required, there are several federal government sources that could be researched.
The USDA-Natural Resources Conservation Service (NRCS) provides access and links to numerous data bases. Categories covered by the NRCS include Earth Cover/Vegetation, Hydrography, 1992 Natural Resources Inventory, and Water and Climate. A final category of Other Data includes an availability map for the National Aerial Photography Program, DEM's, Digital Orthophotography, census links, and U.S. and World Data.
Several files in the NRCS Clearing House are in the PDF format developed by Adobe Systems Incorporated. To view these files requires the Adobe Acrobat Reader which can be downloaded for free from their Web pages. An advantage to this format is it allows a person to "view, navigate, and print PDF files across all major computing platforms."
The Environmental Protection Agency's National GIS Program. This program is well organized. Data for download is housed under the EPA's Spatial Data Library System (ESDLS). The ESDLS is a repository for new and legacy geospatial data in ARC/INFO format. Data housed includes:
- 1992 TIGER files
- 1995 USGS Geographic Names Information System 2 (GNIS2) points
- 1990 Census Block data
- Roads, hydrography, state and county boundary data from 1:2,000,000 DLGs
- Boundaries of National Priority List (NPL-Superfund) Sites
Additionally, eight other data categories are proposed to be added to the ESDLS in the future.
This EPA GIS program also contains a Metadata Clearinghouse for the EPA's Spatial Data. Like many other data clearinghouses, this one is a node of the National Geospatial Data Clearinghouse, a component of the National Spatial Data Infrastructure (NSDI).
"The National Spatial Data Infrastructure (NSDI) encompasses policies, standards, and procedures for organizations to cooperatively produce and share geospatial data. The Federal Geographic Data Committee (FGDC) has assumed leadership in the evolution of the NSDI in cooperation with state and local governments, academia, and the private sector."
Other data sources briefly mentioned earlier in this document include:
- U.S. Census Bureau, (TIGER) files.
- U.S. Geologic Survey's U.S. Geodata downloadable files include DEM's, DLG's, and Land Use Land Cover (LULC) maps. A future site for USGS data files is the Earth Resources Observation Systems (EROS) Data Center .
- Commercial Sources
If all the above data sources don't have the data required and money is no object, then maybe the best solution is to contact one of the many commercial vendors available. Below is by no means a comprehensive listing of commercial vendors, nor is it a ranking of any sort. It is a list of sites I have encountered that may be able to help in your efforts:
Space Imaging EOSAT of Thorton, Colorado offers an Online Products and Services Catalog.
Intermountain Digital Imaging, L.C., of Salt Lake City, Utah.
Mead & Hunt, with offices in Prairie Valley, Kansas, La Crescent , Minnesota, and Green Bay and Madison, Wisconsin.
As I stated, this project is an attempt to provide individuals engaged in geospatial research with a "one-stop-shopping" point of reference for public access data. I briefly described the two predominant data models one will encounter - raster and vector, and mentioned some of their applications. I also gave a quick overview of some of the more common data formats in use. Finally, I ended with a fairly comprehensive listing of sources of data suitable for a geographic information system or geospatially referenced project.
As a final note, if after using any of these sites you encounter a problem, or have a suggestion, please e-mail me at the address in my biography.