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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