Dept. of Geography - Computing Orientation - Fall 1999 GIS Data Sources.
1 GIS Data Sources. 2 GIS Data: Primary Sources Primary data sources –Created “in house”...
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Transcript of 1 GIS Data Sources. 2 GIS Data: Primary Sources Primary data sources –Created “in house”...
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GIS Data Sources
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GIS Data: Primary Sources
• Primary data sources– Created “in house”
• Through your own or your team’s field data collection• By transforming data from sources not yet available
digitally• For use by the same organization
– High level quality control– Often customized for specific
project/application– Costly
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Example of Primary Data: GPS Data Input
• Supports development of highly accurate geodetic control
• Links field data collection to locations
• Cost & accuracy vary
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GIS Data: Secondary Sources
• Secondary data sources– Outside data providers
• Government• Third party vendors
– Transformations and/or conversion often required• Formats (e.g., ASCII to GRID)• Projections• Units of measurements (e.g., feet to meters)
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GIS Data: Secondary Sources (cont.)
• Datasets may be difficult to find• Data costs & sensitivity may limit access• Need to be aware of vendor’s quality
control procedures to be able to judge data quality
• What type of information included about data (i.e., metadata)?– Scale, resolution, field names & descriptions,
codes & meaning– Need enough info to be able to make decisions
about whether data use is appropriate
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Examples of Government Data Providers
• U.S. Census Bureau– TIGER
• U.S. Geological Survey (USGS)– Imagery, DEMs, DRGs, DLGs
• Natural Resource Conservation Service– STATSGO (U.S. General Soil Map)
• National Oceanic & Atmospheric Agency– Coastal management– Oil & chemical spills– Coral reef conservation
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Data Sources: U.S. Census Bureau
• Charged with the Constitutional responsibility of carrying out the decennial census
• Census of Population and Housing
• Very large mapping component involved in undertaking a U.S. national census!
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U.S. Census Data
• The U.S. Census acquires data from households about:– Demographic info– Race– Population– Disabilities– Migration
• Economic info– Living quarters– Occupation– Income
• Social info– Languages spoken– School enrollment– Family structure in the home– Marital status– Ancestry– Foreign-born population
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Making Census Data Spatial
• Census data collected by household
• Geographic location of the household must be known– To find household to deliver questionnaire– To relate the questionnaire to a place
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U.S. Census Data & TIGER/Line Files
• TIGER/Line files:• The “geography” of the census• Topologically Integrated Geographic Encoding &
Referencing• Examples of layers of this massive dataset:
– Streets – Address ranges of households– Landmarks for census-taking– County/city/block boundaries
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U.S. Census Data & TIGER/Line Files
• TIGER designed to: – Support pre-census functions in preparation
for Census of Population and Housing – Support census-taking efforts – Evaluate success of the Census – Provide geographic framework for analysis of
Census data
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Before TIGER/Line
• TIGER/Line files used in the 1990 and 2000 censuses
• Will be used in the future by the Census Bureau
• Before TIGER/Line files was GBF/DIME system• GBF/DIME• Created in 1967• Used for the 1970 and 1980 censuses
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Enumeration Units for TIGER/LINE Files
• Census data are not available to public by household
• Data aggregated to larger geographic units– This is why you used blocks and block groups
in your lab
• The data still allow for useful geographic data summaries
• Prevents breaches of privacy
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• Hierarchical tabulation systems, e.g.:
USARegionDivisionStateCountyTractBlock GroupBlock
• 2000 Census tallies for entire US:
65,443 tracts208,790 block groups8,205,582 blocks
• In North Carolina:1,563 tracts5,271 block groups232,403 blocks
Census Geographic Hierarchy
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TIGER/LINE Files• Supporting geography • Roads/streets/highways • Basic hydrography• Point & area landmarks
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TIGER Area (Polygon) & Landmark Data
Point and polygon landmarks
Census geography (tracts, blocks, etc.) used for reporting Census data
ID linkage from polygons in TIGER/Line data to Census attribute data
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TIGER line and address data
• Roads– Attributes
include:• Basic road type• Address ranges
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TIGER Address Data
• Address ranges: street address numbers at the beginning and the end of all arcs/lines in the database – Allows address geocoding
Match data with address to a spatial location using an interpolated estimate
• Problem: still some incomplete address range data, esp. in rural areas
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Using Census data
• Lets us merge a tremendously rich source of detailed socioeconomic data (Census) with a comprehensive geography for the entire country
Orange County, NC block groups w/ median income data (darker green = higher income)
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Base Maps
• “Background framework on which a thematic map is drawn”
• For example: the background layer on a GIS that provides a frame of reference for the data being displayed
• Can be maps of natural features, human-made objects, or both
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• Also called “base maps”• These are the starting point of a GIS project
– E.G., the background data on which you overlay your GIS analysis results
• Most countries have federal agencies charged with mapping national resources & infrastructure…– E.g., United States Geological Survey (USGS) – established
1879• Many European countries far earlier
– Also previously created by colonial powers for colonies– These topographic “base” maps valuable spatial data
source– Finding and getting access can be difficult if working abroad
• Contact the national agencies
National Topographic Maps
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National Topographic Maps• “Topographic map”
– The name refers to information (contour lines) about terrain & elevation (topography)
– However, “topographic maps” generally contain much more than just elevation info
• Topographic base map info increasingly digital– Many developed countries are well into the conversion
process• U.S. nearly complete
– Less-developed countries much less so
• Even with digital source data we may still need to be able to gain access, import data format, etc. from the original maps
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USGS Topographic Maps
• Complete coverage of the U.S. (not at all scales)
• Large-scale - 1:24,000 – Occasional maps at 1:25,000 (metric)– “7.5-minute quadrangle” map sheets -- 54,000 to cover
contiguous U.S. and Hawaii– Alaska only available as 1:63,360 – Maps include:
• Contour lines, roads, railroads, hydrography, cemeteries, schools, etc…
• Smaller-scale 1:100,000 and 1:250,000 topographic map series– Same basic things on the map, but much less detail
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Digital Versions of USGS Topographic Maps
• USGS topographic maps were used to make several different digital products
• Scales– 1:24,000 (large scale) + 1:63,360 for
Alaska– 1:100,000 (intermediate scale)– 1:2,000,000 (small scale)
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USGS Data: Digital Raster
Graphics (DRG)
• Raster images of standard topographic maps– Scanned and
georeferenced– Pixel attribute =
color
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USGS Data: Digital Line
Graphs (DLG)
• Linear features from standard topographic maps
• May include:– Boundaries (state, county, city,
national parks/preserves) – Hydrography (rivers, streams,
lake shores– Transportation (roads, streets,
railroads)– Elevation contours and spot
elevation values
Image of USGS 1:24,000 West Rapid City (South Dakota) DLG quadrangle.
Hydrography shown in cyan, roads in red,
pipelines/transmission lines in green, & railroads in blue. (GIF image from USGS.)
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USGS Data: Digital
Elevation Models (DEM)
• Raster-format elevation data derived from scanned topographic maps– Elevation samples at
regularly-spaced intervals
– Raster resolution (i.e., cell size) is limited by the scale of the original topographic map
– 1:24,000 Quadrangles were used to produce DEMS with a spatial resolution of 30-meters
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USGS Remote Sensing
• Aerial photography– Source of base map data for many products
check products 1st
– Distortions caused by scale, relief, tilt
• Orthophotos/orthophotoquads (aerial photography)– Digital Ortho Quarter Quadrangles (DOQQ)– Spatial resolution of ~ 1 – meter– Black & White – Corrected for scale, relief, tilt distortion– Available in analog & digital formats
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USGS Remote Sensing• NASA is the agency responsible for developing & launching
the satellites, but USGS helps with distribution
• Many remotely sensed datasets acquired by the US government are available online through EROS (Earth Resources Observation & Science) run by USGS– These data are NOT free– Cost-free DOQQs are available online for most states
• Other types of data that can be found in EROS include– Landsat TM imagery– Color-InfraRed air photography (CIR)
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How Analog Geographic Data Becomes Digital
• For converting:– Maps that were made before GIS– Maps made in the field by hand– Analog photos to digital
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Input Devices• Manual input devices
– Digitizing• Transforms information from analog format (e.g.,
paper, Mylar) digital format for computer storage & display
• Vector data capture • Methods
– Digitizing tablet – On screen digitizing using PC
– GPS• Vector data capture
– Scanners• Vector & raster data capture (depends on scanner
type)
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Input Devices : Small format digitizer
http://www.digitizerpro.com/calcomp.htm
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Digitizing Tablet• Electronically active table surface
– Fine grid of wires acts as a Cartesian coordinate system
– Small & large formats available
http://www.calmit.unl.edu/geog412/Digitizing.pdf
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Digitizing w/ digitizing tablet
http://www.calmit.unl.edu/geog412/Digitizing.pdf
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Digitizing Tablet
• Puck– Connected to tablet– Records locations from map– Crosshair feature locator– Buttons indicate beginning/ending
of lines/polygons, left/right polygons
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Selection & Use of Digitizers
• Qualities to be aware of– Repeatability– Linearity– Resolution– Skew– Stability
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Selection & Use of Digitizers• Repeatability: Precision; expectation that location data recorded
for a single location will be same – Good = 0.001 inch
• Linearity: Measure of digitizer’s ability to be within a specified distance (tolerance) of the correct value as the puck is move over large distance– Common tolerance level: 0.003 in over 60 in
• Resolution: Digitizer’s ability to record increments of space– Smaller value higher resolution
• Stability: Tendency of reading to change as digitizer warms up
• Skew: Do the results produced have the intended shape?– Rectangular coordinates input rectangular output– Some portions of the tablet can wear out
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Input devices: Scanners
• Types:– Line-following vector output
• Placed on line, moves on small wheels– Requires technician
• Distance/time intervals dictate coordinates recorded
– Problem when line is complex• Can get confused (convergence/divergence, color
contrast)
– Flatbed raster output– Drum scanners raster output
• Automated but edits require user intervention
39http://www.liv.ac.uk/abe/students/photoshop/images/f05_scanner.jpg
Flatbed scanner & CCD
• Inexpensive & commonly available
• Use CCD (charge-coupled device)
• Output: raster image– Can be converted to
vector
CCD
http://www.nortekonline.com/eng/Product/
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Input devices: Drum scanner
• Scans one line at a time• Drum rotates & sensor moves
perpendicular to direction of rotation• Can take longer maps than flatbed• Output: raster image
– Can be converted to vector
From Fundamentals of Geographic Information Systems, Demers (2005)
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Making New Geographic Datasets Manually From Existing Digital Data
• Often done by manually “drawing” features on air photos or satellite imagery on screen
• Can be used for small areas or in cases where a computer algorithm doesn’t adequately map the features of interest
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Also called “heads-up” digitizing
On-screen digitizing w/ PC
http://www.esri.com/news/arcnews/winter0102articles/epas-clean-water.html