1. GIS Data Sources

2. 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

3. Example of Primary Data: GPS Data Input
Supports development of highly accurate geodetic control
Links field data collection to locations
Cost & accuracy vary

4. 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)

5. 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

6. 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

7. 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!

8. 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

9. 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

10. U.S. Census Data & 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

11. 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

12. 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

13. 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

14. Census Geographic Hierarchy
Hierarchical tabulation systems, e.g.:
USA
Region
Division
State
County
Tract
Block Group
Block
2000 Census tallies for entire US:
65,443 tracts
208,790 block groups
8,205,582 blocks
In North Carolina:
1,563 tracts
5,271 block groups
232,403 blocks

15. TIGER/LINE Files Supporting geography Roads/streets/highways
Basic hydrography
Point & area landmarks

16. 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

17. TIGER line and address data
Roads
Attributes include:
Basic road type
Address ranges

18. 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

19. 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)

20. 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

21. National Topographic Maps
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

22. National Topographic Maps
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

23. 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 ,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

24. 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)

25. USGS Data: Digital Raster Graphics (DRG)
Raster images of standard topographic maps
Scanned and georeferenced
Pixel attribute = color

26. 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.)

27. 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

28. 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
Scale – a photograph over variable terrain represents a range of different scales (object higher in elevation appears bigger than same-sized object at lower elevation)
Tilt – aircraft perspective-induced distortion in objects closer due to tilt
Relief – Relief > 0.5% of flying height causes distortion
Orthophotoquads – based on areas occupied by topo quads

29. 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)

30. 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

31. Input Devices Manual input devices Digitizing GPS Scanners
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
Scanners
Vector & raster data capture (depends on scanner type)

32. Input Devices : Small format digitizer

33. Digitizing Tablet Electronically active table surface
Fine grid of wires acts as a Cartesian coordinate system
Small & large formats available
Cartesian coordinate system: A two-dimensional, planar coordinate system in which x measures horizontal distance and y measures vertical distance. Each point on the plane is defined by an x,y coordinate.
--As opposed to the spherical grid system that serves as the basis for lat/long measurements

34. Digitizing w/ digitizing tablet

35. Digitizing Tablet Puck Connected to tablet Records locations from map
Crosshair  feature locator
Buttons  indicate beginning/ending of lines/polygons, left/right polygons

36. Selection & Use of Digitizers
Qualities to be aware of
Repeatability
Linearity
Resolution
Skew
Stability

37. Selection & Use of Digitizers
Repeatability: Precision; expectation that location data recorded for a single location will be same
Good = 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: 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

38. 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
A charge-coupled device (CCD) is an image sensor, consisting of an integrated circuit containing an array of linked, or coupled, capacitors sensitive to light.
CCDs containing grids of pixels are used in digital cameras, optical scanners and video cameras as light-sensing devices.
Can capture the image 1 line at a time or wholly
An image is projected by a lens on the capacitor array, causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbour. The last capacitor in the array dumps its charge into an amplifier that converts the charge into a voltage. By repeating this process, the control circuit converts the entire contents of the array to a varying voltage, which it samples, digitizes and stores in memory.

39. Flatbed scanner & CCD Inexpensive & commonly available
Use CCD (charge-coupled device)
Output: raster image
Can be converted to vector
CCDs: commonly used in aerial photography, satellite remote sensing, and digital cameras
Light is converted into a pattern of electronic charges

40. 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)

41. 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

42. On-screen digitizing w/ PC
DOQ used in image above.
DOQ: A photographic image that has been converted to raster form and horizontally corrected for the effects of relief displacement, aircraft tip and tilt, and other image distortions and displacements. It is a photographic map stored in a digital computer environment.
Also called “heads-up” digitizing