Digital Data

1. Digital Elevation Models
        National Cartographic Information Center (NCIC) of USGS

        1:24,000 DEM

        1:250,000 DEM

        The derivatives of the two basic DEM data

2. Census Data
        Census units
        - Census tracts
                defined by the U.S. Census Bureau for statistical
                        reporting of demographic, economic, and other data
                their boundaries follow major streets or natural features
                each tract comprise a population of approximately 4,000
                census tracts are numbered consecutively and unique
                        within the jurisdiction

        - Block groups
                comprise a population of approximately 1,000
                boundaries coterminous with census tract boundaries

        - Block
                smallest geographic area formed by street segments
                coded by 3 digit numbers, the first one represents the
                block group, the last two are consecutively numbered
                and unique within the block group

        GBF/DIME Files (Census Bureau for 1970, 1980 census)
           - Geographic Base Files (GBF)
                computerized data files that contain addresses, census
                tracts, x and y coord, political boundaries, etc.

           - Dual Independent Map Encoding system (DIME) -
                developed by the Bureau of Census for 1970, 1980 census
                topologically structured

        Topologically Integrated Geographic Encoding and Referencing
        (TIGER) Files
        - Developed by the US Census Bureau for 1990 census
                the spatial and attribute data were combined into one
                system, and it is topologically structured

        - geographic features:
                DIME files and USGS 1:100,000 Digital Line Graph (DLG)
                geographic area codes (county, city, census tract, block
                numbers, etc.)
                roads, hydrography, railroads, utilities, and transportation

        - attributes:
                feature names, address range and ZIP codes for areas
                overlap with 1980 GBF/DIME files

        - additional attributes: population, housing unit counts,
                income, occupation, racial distribution, housing values,
                etc.

        - block level contains aggregated information

3. Digital Line Graph (DLG)
        Base cartographic data, distributed by NCIC of USGS
        - Topographic data - data that portray relief. e.g. DEM
        - Planimetric data - data that include transportation,
           hydrography, and administrative and political boundaries
        - include spatial data, attribute data, and topology

        - Data content:                         1:24,000;      1:100,000;      1:2,000,000
                                                                7.5'x7.5'         30'x 30'        multi-state
          boundaries                                       x                        x                      x
               state, county, city, park
          hydrography                                    x                         x                      x
            streams and water bodies
          public land survey system                 x                         x
            rectangular system
          transportation                                  x                         x                       x
            roads/trails,railroads, pipelines,
          other mandate structures                  x
            ski lift
          hypsography                                    x                         x                       x
            contour lines 
          surface cover                                   x
            vegetative
          non-vegetative surface cover            x
            sand, gravel, etc.
          survey control and markers               x
            positions and elevations

        - Attribute codes
        major vs. minor codes

        The derivatives of the DLG data

4. Land Use and Land Cover Digital Data (LULC)
          distributed by USGS NCIC
                1:250,000 - 1⁰x2⁰
                1:100,000 - 30'x60'

        - Data content
          land use and land cover
                Anderson classification system level II
          political unit
                state, county, and independent city boundaries
          census county subdivision
                census tracks for MSA, or minor civil divisions
          hydrologic unit
                8 digits HUC, region, subregion, acct. cataloging
          federal land ownership

          Later versions of LULC data and remotely sensed images

5. Soils Data
        - developed by USDA Natural Resource Conservation Service
            (NRCS)
        Soil Survey Geographic Data Base (SSURGO)
                1:24,000, 7.5x7.5 minute quads
                based on aerial photographs and field survey

                map unit and soil classification: soil series
                attributes: physical, chemical, biological, land use

        State Soil Geographic Data Base (STATSGO)
                1:250,000, 1x2 degree
                map unit: soil association
                attribute: 60 soil properties
                attribute structure
                        map unit and code
                        components and their properties
                        layers and their properties

   National Soil Geographic Data Base (NATSGO)
                1:7,500,000
   
  6. Digital Orthorphoto Quarter Quad (DOQQ)
         - developed by USGS and state and local governments
            Digital aerial photographs without geometric errors introduced by
            tilt or relief displacement

7. Digital Raster Graphics (DRG)
        - developed by USGS

          
 
http://cugir.mannlib.cornell.edu/
http://www.nysgis.state.ny.us/
http://seamless.usgs.gov/
http://datagateway.nrcs.usda.gov/

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