Introduction to Geographic Information Systems
Spring 2013 (INF 385T-28437)
Dr. David Arctur
Lecturer, Research Fellow
University of Texas at Austin
Lecture 5
February 7, 2013
Spatial Reference Systems,
Data Sources

Outline
 Models of the Earth
 Map projections
 Coordinate systems
 GIS data sources
 Vector data formats
 Raster data formats
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Models of the Earth’s shape

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



The Geoid is approximately where sea level would be throughout the world
(measured by plumb bob away from coastal areas)
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

Horizontal Control Datums
Commons North American Datums

NAD27 (1927 North American Datum)
 Clarke (1866) ellipsoid, non-geocentric (local origin) for axis of rotation

NAD83 (1983 North American Datum)
 GRS80 ellipsoid, geocentric origin for axis of rotation

WGS84 (1984 World Geodetic System)
 WGS84 ellipsoid, geocentric, nearly identical to
NAD83

Other datums are also in use globally
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Datum shifts
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Datum transformations

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
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Latitude and longitude

Longitude (meridians)
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Latitude and longitude

Latitude (parallels)
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Latitude and longitude
0 ° Longitude (prime meridian)
0 ° Latitude (equator)
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Latitude and longitude

Coordinates
Pittsburgh, PA USA
40
-80
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Lat/Long coordinates

Degrees, minutes, and seconds (DMS):
 40 ° 26 ′ 2″ N latitude
 -80 ° 0 ′ 58″ W longitude

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
Translated to distance

World circumference through the poles is
24,859.82 miles , so for latitude:
 1 ° = 24,859.82 / 360 = 69.1 miles
 1′ = 24,859.82 / (360 * 60) = 1.15 miles
 1″ = 24,859.82 * 5,280 / (360 * 3,600) = 101 feet

Length of the equator is 24,901.55 miles
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Picking a projection …
[or: how big do you like Greenland?]
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Most-used methods
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Mercator projection (1569)
 Conformal projection
 Cylindrical
 Parallels and meridians at right angles
 Linear scale is constant in all directions around any point
 Preserves angles and shapes of small objects
 Distorts the size and shape of large objects
 Map projection for nautical purposes
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Hammer – Aitoff (1882-1889)
 Equal-area
 Modified azimuthal projection
 Good for population density (world area)
 Difficult to see some areas
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Robinson projection (1961)
 Pseudocylindrical
 Neither equal area nor conformal
 Meridians curve gently, avoiding extremes
 Good compromise projection for viewing entire world
 Used by Rand McNally since the 1960s and by the
National Geographic
Society (1988 and 1998)
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Albers Equal Area
 Conic projection
 Scale and shape are not preserved, distortion is minimal between the standard parallels
 Standard projection for
British Columbia,
U.S. Geological Survey,
U.S. Census Bureau
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Other map projections… http://www.watermanpolyhedron.com/maps
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And the everpopular…
Spilled
Coffee
Projection
Bovine projection(s)
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Projection important for…
 Measurements used to make important decisions
 Comparing shapes, areas, distances, or directions of map features
 Feature and image themes are aligned
New York
New York
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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Projection not important for…

Business applications
 Not of critical importance
 Concerned with the relative location of different features

Large scale maps — street maps
 Distortion may be negligible
 Map covers only a small part of the earth’s surface
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Lecture 5
COORDINATE SYSTEMS
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Geographic Coordinate System
(GCS)
 Spherical coordinates
 Angles of rotation of a radius anchored at earth’s center
 Latitude and longitude
 Census Bureau
TIGER files
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U.S. Census GCS example
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Rectangular coordinate system
 Used for locating an intersection on a flat sheet of graph paper or a flat map
 Cartesian coordinates (x,y)
 State plane and
UTM
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State Plane coordinates
 Established by the
U.S. Coast and Geodetic
Survey in 1930s
 Originally North American
Datum (NAD 1927)
 More recently NAD 1983 and
1983 HARN
 Used by local U.S. governments
 All positive coordinates in feet
(or meters)
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State Plane zones
 125 zones

At least one for each state

Cannot have zones joined to make larger regions
 Follow state and county boundaries
 Each has its own projection:

Lambert conformal projection for zones with east-west extent

Transverse Mercator projection for zones with north-south extent
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State Plane zones
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State Plane zones
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Pittsburgh neighborhoods as state plane coordinates
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Universal Transverse Mercator (UTM)
 Rectangular coordinate system
 Used by U.S. military
 Covers entire world
 Metric coordinates
 Longitude zones are 6 ° wide
 Latitude zones are 8 ° high
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Coordinate system summary

Geographic coordinate system
 U.S. Census

State plane coordinate system
 Local governments
 U.S. military

Projections defined in ArcCatalog or ArcMap
(.prj) files

First file added in a map document sets the projection (others will adjust to it as long as they have a .prj file)
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We had to go through all that, so we can understand issues around importing spatial data from…
GIS DATA SOURCES
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GIS data sources

ESRI

U.S. Census

USGS and other government sources

GDT Dynamap/2000 U.S. Street Data



Engineering companies
 land surveys, aerial photos, CAD drawings
University Web sites (e.g. Penn State’s
PASDA)
Zillions of others…
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GIS data sources

30+ million Internet search results
 type “GIS data download” or “population China
.e00
 add the name of the state, county, or city to the search
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GIS departments Web sites

Washington, D.C.
 dcgis .
dc .gov/

Chicago, IL
 www.cityof
chicago .org/ gis

Austin, TX
 Tip: Search by county name (Travis County, Texas)

 http://www.austintexas.gov/development/ ftp://ftp.ci.austin.tx.us/GIS-Data/Regional/coa_gis.html
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ESRI’s Web site
 http://www.esri.com/data/esri_data/demographic-overview
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U.S. Census Bureau

Started building a map infrastructure in the late 1970s and early 1980s

Census mapping needs were twofold:
 To assign census employees to areas of responsibility, covering the entire country and its possessions
 To report and display census tabulations by area, officials determined that the smallest area needed for these purposes is a city block or its equivalent
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U.S. Census Bureau

Compiles all line features used to create a block layer for the entire country

Map features smaller than are the responsibility of local governments
 deeded land parcels
 buildings
 street curbs
 parking lots
 others?
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Census TIGER/Line files

T opologically I ntegrated G eographic
E ncoding and R eferencing files
 Census Bureau’s product for digital mapping of the U.S.
 Available for the entire U.S. and its possessions
 Include the following geographic features
 roads and street centerlines
 railroads
 rivers
 lakes
 census statistical boundaries
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TIGER census tracts

Statistical boundary (below county level)
 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 nonvisible features
 more than 60,000 census tracts in Census 2000

Also, the legal basis for developing congressional districts
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PA tracts
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Allegheny County tracts
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Pittsburgh tracts
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TIGER census block groups

Subdivision of a census tract
 400 housing units, with a minimum of 250 and a maximum of 550 housing units
 Follow clearly visible features such as roads, rivers, and railroads
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Census block groups
GIS TUTORIAL 1 - Basic Workbook
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TIGER census blocks

Smallest geographic area for which the
Census Bureau collects and tabulates decennial census information
 Visible boundaries
 street
 road
 stream

Shoreline
 Nonvisible boundaries
 county, city, neighborhood boundary
 property line
GIS TUTORIAL 1 - Basic Workbook
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Census blocks
GIS TUTORIAL 1 - Basic Workbook
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Other TIGER layers
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U.S. Census Bureau data tables
 http://factfinder2.census.gov/
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Summary File (SF1) tables
GIS TUTORIAL 1 - Basic Workbook
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Summary File (SF3) tables
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SF tables comparisons
SF1
 Population
 Age
 Sex
 Race
 Housing units
 FFH
SF3
 Income
 Educational attainment
 Citizenship
 Transportation
 Detailed housing
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Census summary

Shapefiles downloaded from www.census.gov or www.esri.com

Data tables downloaded from American
Factfinder http://factfinder2.census.gov

Data joins needed to join SF1 or SF3 to shapefiles
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Lecture 5
VECTOR DATA FORMATS
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ArcInfo coverages




Created using ESRI’s ArcInfo software
Older format (import/export as “.e00”)
Set of files within a folder or directory called a workspace
Files represent different types of topology or feature types
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Coverage attribute table

Area and perimeter
 Coverage_ and Coverage_ID
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Shapefiles

ArcView native format
 Minimum files

.shp
–stores feature geometry

.shx
–stores index of features

.dbf
–stores attribute data
 Additional files

.prj
–projection data

.xml
–metadata

.sbn and .sbx
–store additional indices
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CAD drawings

CAD software
 Autodesk, AutoCAD (.dwg)
 Bentley, Microstation (.dgn, .dxf)

Often used by engineering companies
 Architectural details, instructions to builders
 Roads, bridges, dams

Better digitizing precision
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CAD drawings
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CAD layers
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Event files
Data table that includes map coordinates, such as latitude and longitude or projected coordinates
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Event files
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Export event files

Creates point features
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Lecture 5
RASTER DATA FORMATS
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Digital file formats

TIFF (Tagged Image File Format)
 .tif file extension
 Very high quality images
 Commonly used in publishing
 Sizes are large because it is uncompressed

GIF (Graphic Interchange Format):
 .gif as its file extension.
 Ideal for schematic drawings that have relatively large areas with solid color fill and few color variations.
 Small file sizes
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Digital file formats

JPEG (Joint Photographic Experts Group):
 .jpg file extension.
 Most widely used format for photographs and other images that have a lot of color variations
 Uses file compression at the expense of picture detail, if you specify a lot of compression
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Summary
 Models of the Earth
 Map projections
 Coordinate systems
 GIS data sources
 Vector data formats
 Raster data formats
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