# lecture 5 ppt

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

(1927 North American Datum)

Clarke (1866) ellipsoid, non-geocentric (local origin) for axis of rotation

(1983 North American Datum)

GRS80 ellipsoid, geocentric origin for axis of rotation

WGS84

(1984 World Geodetic System)

WGS84 ellipsoid, geocentric, nearly identical to

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

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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…

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

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

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

.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

 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

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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

 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

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

.prj

–projection data

.xml

.sbn and .sbx

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Bentley, Microstation (.dgn, .dxf)

Often used by engineering companies

Architectural details, instructions to builders

Better digitizing precision

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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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