Introduction to Geographic Information Systems
Fall 2013 (INF 385T-28620)
Spatial Reference Systems, Data Sources
Dr. David Arctur
Research Fellow, Adjunct Faculty
University of Texas at Austin
Lecture 5
September 19, 2013
Outline
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Models of the Earth
Map coordinates
Map projections
US Census geographic files
US Census data files
Geospatial data sources
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Models of the Earth’s shape
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Sphere with radius of ~6378 km
Ellipsoid (or Spheroid) with equatorial radius
(semimajor axis) of ~6378 km and polar
radius (semiminor axis) of ~6357 km
 Difference of ~21km usually expressed as
“flattening” (f) ratio of the ellipsoid:
 f = difference / major axis = ~ 1/300 for Earth
 and “inverse flattening” would be ~300
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Ellipsoid dimensions and flattening
Ellipsoid = Spheroid in GIS…
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Ellipsoid vs Geoid vs Datum
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The Geoid is approximately
where sea level would be
throughout the world
(measured by plumb bob away
from coastal areas)
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Due to variations in the
Earth’s gravity field, this
“global sea level” would not
fit any one ellipsoid, as
evident in figure 
Datum = shape of ellipsoid
AND location of origin for
axis of rotation relative to
Earth center of mass
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Horizontal Control Datums
Commons North American Datums
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NAD27 (1927 North American Datum)
 Clarke (1866) ellipsoid, non-geocentric (local origin)
for axis of rotation
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NAD83 (1983 North American Datum)
 GRS80 ellipsoid, geocentric origin for axis of rotation
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WGS84 (1984 World Geodetic System)
 WGS84 ellipsoid, geocentric, nearly identical to
NAD83
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Other datums are also in use globally
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Datum shifts
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Datum transformations
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Theoretical method: use equations relating
Lat/Lon in one datum to another
Empirical method: use grid of differences to
convert values directly from one datum to
another
See Esri digital book on Map Projections for
more information
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How do we get from 3D Earth to 2D maps???
MAP PROJECTIONS
Map projections
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Way to represent the curved surface of the
earth on the flat surface of a map
Hundreds of map projections
Each map projection has advantages and
disadvantages:
 Depends on the scale of the map
 Depends on map’s purpose
 Different projections good for small areas, areas
with a large east–west extent, or areas with a
large north–south extent
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Picking a projection …
[or: how big do you like Greenland?]
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Map projections
Flatten half of a rubber ball?
No. Instead, features are projected onto one of three “developable”
surfaces.
Planar: a map projection
resulting from the
conceptual projection of
the earth onto a tangent or
secant plane
Cylindrical: a map projection where the
earth’s surface is projected onto a
tangent or secant cylinder, which is then
cut lengthwise and laid flat
Conic: a map projection where the earth’s
surface is projected onto a tangent or secant
cone, which is then cut from apex to base and
laid flat
http://www.nationalatlas.gov/articles/mapping/a_projections.html#two
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Most-used methods
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Conformal projection
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Cylindrical projection
Parallels and meridians at
right angles
Angles and shapes of small
objects preserved (at every
point, east–west scale same
as north–south scale)
The size/shape/area of large
objects distorted (scale
approaches infinity at the
poles)
Seldom used for world maps
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Example: Mercator projection (1569)
used for nautical purposes (constant
courses are straight lines)
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Equivalent projection
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Conic projection
Preserves accurate area
Scale and shape are not
preserved
Example: Albers Equal Area
standard projection for US Geological
Survey, US Census Bureau
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Compromise projections
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Neither equivalent nor
conformal
Meridians curve gently,
avoiding extremes.
Doesn’t preserve
properties, but “looks
right”
Example:
Robinson projection (1961)
• good compromise projection for viewing
entire world
• used by Rand McNally and the National
Geographic Society
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And the ever-popular…
Spilled
Coffee
Projection
Bovine projection(s)
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When projection is important
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Small-scale maps
 Comparing shapes, areas, distances, or directions of map
features
 Natural appearance desired
New York
New York
Los Angeles
Los
Angeles
Los Angeles
Projection: Mercator
Distance: 3,124.67 miles
Projection: Albers Equal Area
Distance: 2,455.03 miles
Actual distance: 2,451 miles
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When projection is not important
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Many business, policy, and management
applications
On large-scale maps
 Error is negligible
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Now here, know where, or nowhere?
MAP COORDINATES
Latitude and longitude
0 ° longitude (prime meridian)
0 ° latitude (equator)
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Latitude and longitude
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Coordinates
Pittsburgh, PA USA
40
-80
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Lat/Long coordinates
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Degrees, minutes, and seconds (DMS)
 40° 26′ 2″ N latitude
 -80° 0′ 58″ W longitude
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Decimal degrees (DD)
 1 degree = 60 minutes
 1 minute = 60 seconds
40° 26′ 2″ =
40 + (26 / 60) + (2 / 3600) =
40 + .43333 + .00055 =
40.434°
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Lat/long coordinates
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Translated to distance
 World circumference through the poles is
24,859.82 mi, so for latitude:
 1° = 24,859.82 / 360 = 69.1 mi
 1′ = 24,859.82 / (360 * 60) = 1.15 mi
 1″ = 24,859.82 * 5,280 / (360 * 3,600) = 101 ft
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Length of the equator is 24,901.55 mi
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GCS example (census tracts)
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Rectangular coordinates
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UTM (universal transverse Mercator)
 US military
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State plane
 Local US governments
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UTM coordinates example
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Developed by US
Army Corps of
Engineers (1940s)
Covers world, 80°S
to 80°N
Metric coordinates
60 tuned transverse
Mercator projections
for longitude zones,
6° wide
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State plane coordinates
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Established by the
US Coast and Geodetic Survey in the 1930s
All positive coordinates in feet or meters
Used by local US governments
Originally North American Datum (NAD
1927)
More recently NAD 1983 and 1983 HARN
(High Accuracy Reference Network)
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State plane zones
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125 zones
 At least one for each state
 Cannot join zones to make larger regions
 Follow state and county boundaries
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Each zone has its own tuned projection
 Lambert conformal projection for zones with
eastwest orientation
 Transverse Mercator projection for zones with
northsouth orientation
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State plane zones
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State plane coordinates example
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State plane NAD 1983, Pennsylvania South, Feet
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X,Y coordinate tips
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Always assign coordinates according to the agency
US Census
City of Pittsburgh
Geographic coordinate system (GCS)
State plane coordinate system
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X,Y coordinate examples
US Census
Geographic coordinates (GCS)
Block groups
City of Pittsburgh
State plane coordinates
Sidewalks
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Map document tip
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The first layer added in ArcMap sets the x,y
coordinate system for the data frame
Additional layers will overlay properly as
long as the correct coordinate system is
assigned to feature class
 For example, GCS to US Census files, state
plane to local government files
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Known as .prj files
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Map document tip
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Example: Sidewalks added first (state plane), but block groups
match even though they are in geographic coordinate system
(GCS) projection.
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Lecture 5
US CENSUS GEOGRAPHIC
FILES
Census TIGER/Line files
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http://www.census.gov/geo/www/tiger/
Topologically Integrated Geographic
Encoding and Referencing files
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US Census Bureau product for digital mapping
of the United States
TIGER maps available for the entire United
States and its possessions, including roads
and streets, railroads, rivers, lakes, political
boundaries, and census statistical boundaries
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Example census geographies
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TIGER census tracts
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Between 1,000 and 8,000 people (in
general)
1,700 housing units or 4,000 people
Homogeneous population characteristics
(economic status and living conditions)
Normally follow visible features
May follow governmental unit boundaries
and other invisible features
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State tracts (2010)
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County tracts (2000 and 2010)
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City tracts (2000 and 2010)
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City block groups (2000 and 2010)
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Subdivisions of a census tract
400 housing units, with a min. of 250 and a max. of 550
Follow clearly visible features (roads, rivers, and railroads)
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Census blocks
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Smallest geographic areas for which the Census Bureau
collects and tabulates decennial census information
Block boundaries visible (street, road, stream, shoreline,
etc.) or invisible (county line, city limit, property line, etc.)
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Lecture 5
US CENSUS DATA FILES
Decennial census data
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Years 2000 and 2010
 Summary File 1 (SF 1)
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Short form, entire population
Population
Age
Sex
Race
Families
Households
Housing units
 Tracts, block groups, blocks
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Decennial census data
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Year 2000 Summary File 3 (SF 3)
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Long form, 1 in 6 households, random
Income, poverty
Educational attainment
Citizenship
Employment, workplace, disability
Transportation, travel time to work
Detailed housing attributes, housing value, residency five
years previous
Languages spoken, ancestry
 Tracts, block groups, NOT blocks
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American Community Survey (ACS)
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Replaces long-form questionnaire and SF3
data
Randomly selects about 3 million
addresses each year to participate
Has rolling, 1-, 3-, and 5-year estimates and
90% confidence intervals
 Add and subtract Margin of Error (MOE) to/from
Estimate to get the confidence interval
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ACS Data
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Age
Sex
Race
Family and relationships
Income and benefits
Health insurance
Education
Veteran status
Disabilities
Where you work and how you get there
Where you live and how much you pay for certain
essentials
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ACS 1-year estimates
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Most current
Data with populations 65,000+
Smallest sample size
Less reliable than 3–5 year
Best used when currency is more important
than precision, or when analyzing large
populations
Not available for tracts or block groups
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ACS 3-year estimates
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Data with populations 20,000+
Larger sample size than 1-year
More reliable than 1-year but less reliable
than 5-year
Best used when analyzing smaller
populations or geographies not available for
1-year estimates
Not available for tracts or block groups
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ACS 5-year estimates
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Data for all areas (tracts and block groups)
Largest sample size
Most reliable but least current
Best used when analyzing small populations,
or when precision is more important than
currency
2005–2009, 2006–2010, etc.
 Note: 2006–2010 only available for county, city, town,
place, American Indian Area, Alaska Native Area, Hawaiian
Home Land, and tracts. Block group estimates are
available only in the ACS Summary File.
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Downloading block group data
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http://www.census.gov/acs/www/data_documentation/summary_file/
Find the tables of interest and their sequence number in the
"Sequenced Number and Table Number" spreadsheet
(http://www2.census.gov/ acs2010_5yr/summaryfile/)
Download the sequences that contain those tables
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Other census data
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Economic census
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Population estimates
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Annual economic surveys
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DataFerret
 http://dataferrett.census.gov/
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Lecture 5
GEOSPATIAL DATA SOURCES
Spatial data infrastructure
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Federal Geographic Data Committee (FGDC)
 This nationwide data publishing effort known as
National Spatial Data Infrastructure (NSDI).
 Established by presidential order
 Responsible for standards, policies, web portals
 FGDC activities are administered through the
FGDC Secretariat, hosted by the US Geological
Survey
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Spatial data packaging
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Metadata
 Documentation enabling intelligent use and interpretation
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Data contents
 Provided by geographic area (political, statistical, tile) or
seamlessly (with extraction by area)
 Quality of geographic features
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Vector maps are generalized for small-scale maps
Raster maps vary by pixel size (30m to a few inches) and
color depth 8 bits to 24 bits per pixel
 Coordinate system
 File format
 Download or web service
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Classification of map layers
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Earth as a system
 Living things are on, under, or above the Earth’s surface
 They depend on the Earth and its environment for life and well-being
 They are organized in political, social, territorial, and other
arrangements
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Map layers
 Physical features:
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Earth’s surface and subsurface
 Environmental features:
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atmosphere, climate, and weather
 Living thing populations:
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people, animals, plants, and microbes
 Organizational features:
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political, legal, administrative, and ecosystem
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National Map orthoimagery
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http://nationalmap.gov/viewer.html
Replacing the digital orthophoto
quadrangles
 High-resolution, seamless
images in UTM coordinates
 Rectified to remove
distortions
 1m resolution with 0.5 m or 1
ft in urban areas, natural
color
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National Elevation Data (NED)
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http://ned.usgs.gov/
 Replaces the digital
elevation model (DEM)
 Seamless raster map
with 30m resolution for
nation and 10m or
better in some areas
Hillshade NED map for Rockville, MD
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Land cover
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http://nationalmap.gov/viewer.html/
 Natural and man-made surface
features
 Collected from satellites in 1992,
2001, and 2006
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National Hydrography Dataset
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http://nhd.usgs.gov/
 Water bodies, lines, and
points
 Identifies segments
(reaches) with network
coding (flow and
direction)
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USGS national water datasets
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http://waterdata.usgs.gov/nwis/rt
 Streamflow conditions
 5,000 stream gages with telemetry
transmits depth
 Program estimates flow rate
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Example geospatial sources
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Government websites (examples)
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http://data.gov/
http://www.geoplatform.gov/home/
http://nationalatlas.gov/
http://nces.ed.gov/ccd/ - National Center for
Education Statistics
Universities
State clearinghouses
Local GIS departments
Libraries
 For example, online business databases
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Example geospatial sources
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Commercial resources
 (Esri, Google, engineering companies, etc.)
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Historic GIS websites
 http://www.nhgis.org/
 http://www.aag.org/cs/projects_and_programs/hi
storical_gis_clearinghouse
 http://peoplemaps.esri.com/pittviewer/
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Summary
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Models of the Earth
Map coordinates
Map projections
US Census geographic files
US Census data files
Geospatial data sources
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