Sources of GIS data

1
Sources of GIS data

• J426 Class 16
• March 5, 2009

2
Overview

• Using existing digital geographic data
• TIGER files
• Issues in using existing digital geographic data
• Using digital attribute data
• Census data
• Issues in using digital attribute data
• Errors in data

3
Using existing digital geographic data

• Increasing amounts of geographic data already
  available in digital form
• Federal agencies are an important source,
  especially U.S. Geological Survey
• Greater amounts of digital geographic data
  becoming available over the Internet
• Links to numbers of sources on Resources page

4
Geographic data from commercial vendors

• Increasing amounts of data available from
  commercial vendors
• Much of it based on U.S. government data
• Value added
• File format for GIS software
• Enhanced accuracy
• More current information
• Example Tele Atlas, GDT street networks

5
TIGER files

• Developed by Bureau of Census
• Have played major role in making GIS available
  and affordable
• Created to support census operations
• Created by Bureau of Census with U.S. Geological
  Survey
• Available on CDROM, Internet

6
TIGER files (contd)

• 1100,000 topographic maps digitized for
  reference information
• Accuracy now being improved
• Contain physical features such as roads,
  railroads, rivers, and landmarks
• Include census statistical units and political
  boundaries
• Include data down to the street level, with
  address ranges

7
TIGER file features
8
Census tract boundaries downloaded from ESRI
9
Issues in using existing digital geographic data

• Data can come in variety of file formats
• Shapefiles (.shp) most common
• ArcGIS can read data in many of the available
  vector and raster data formats
• Some files (including original TIGER files) may
  require separate conversion software

10
Issues in using existing digital geographic data

• Data files may be in various coordinate systems
  (geographic or projected)
• Need to determine coordinate system, add
  projection information or convert

11
Using digital attribute data

• Much data available for census reporting units
• Census Bureau is obviously major source
• Increasing amounts available from other
  governmental units
• Example vital statistics for census tracts from
  Marion County Health and Hospital Corporation

12
Census data

• Most important source of attribute data
• Decennial census of population and housing
• Collected for persons and housing units
• Full count and sample data
• Aggregated to geographic units to protect
  confidentiality
• Available on from Census website

13
Types of census data

• Population
• Age
• Race
• Ethnicity
• Household type
• Education
• Employment and occupation
• Income

• Housing
• Occupancy
• Type
• Age
• Facilities
• Value and rent and other financial information

14
Basic census geography

• United States
• Regions
• States
• Counties
• Census tracts
• Block groups
• Blocks

15
Census attribute datamedian family income by
tractdownloaded from census
16
Issues in using digital attribute data

• Data in most data file formats can be converted
  to formats used by ArcGIS
• .dbf files most common format
• Need field with identifier identical to
  geographic file for joining attribute data
• STFID (based on U.S. FIPS codes) commonly used
  with census data

17
Errors in data

• No such thing as error-free data
• Important to understand nature, source of data
• Early Minnesota GIS found perfect correlations
  between characteristics
• One agency derived data from data collected by
  another agency

18
Sources of geographic error

• Instability of maps
• Errors in base maps
• Errors in originally recording data
• Errors in interpretation, as with air photos
• Imprecise nature of boundaries
• Transition zones in soil types
• Variability of shorelines

19
Importance of scale

• Accuracy related to scale of maps or scale at
  which data were collected and entered
• In GIS, maps can be displayed at any scale
• But scale of source information an important
  indicator of accuracy
• Trying to combine information collected and
  entered and widely-different scales can lead to
  problems

20
Scale and accuracy

• Required accuracy depends on application
• For natural resource data, 124,000 is a common
  target
• At this scale, 0.5 mm line width 12 m on ground
• USGS topographic data in TIGER files is from
  1100,000 maps
• At this scale, 0.5 mm line width 50 m on ground

21
Metadata

• Data about data
• Information about particular data set
• Who created data set and how
• Date of creation
• Accuracy standards employed
• Increasingly available
• Basis for assessing appropriateness of data for
  particular application