introduction to geographic information systems fall 2013 (inf 385t-28620) spatial reference systems,...

Introduction to Geographic Information Systems Fall 2013 (INF 385T-28620)

Spatial Reference Systems, Data Sources Dr. David Arctur

Research Fellow, Adjunct FacultyUniversity of Texas at Austin

Lecture 5September 19, 2013

Outline

Models of the Earth Map coordinates Map projections US Census geographic files US Census data files Geospatial data sources

2INF385T(28620) – Fall 2013 – Lecture 5

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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

INF385T(28620) – Fall 2013 – Lecture 5

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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 massINF385T(28620) – Fall 2013 – Lecture 5

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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


MAP PROJECTIONSHow do we get from 3D Earth to 2D maps???

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Map projections 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?]


Map projectionsFlatten half of a rubber ball?

No. Instead, features are projected onto one of three “developable” surfaces.

http://www.nationalatlas.gov/articles/mapping/a_projections.html#two

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

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

Planar: a map projection resulting from the conceptual projection of the earth onto a tangent or secant plane


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Most-used methods


Conformal projection 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

Example: Mercator projection (1569) used for nautical purposes (constant courses are straight lines)


Equivalent projection 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 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…


Bovine projection(s)

Spilled Coffee

Projection

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Small-scale maps Comparing shapes, areas, distances, or directions of

map features Natural appearance desired

New YorkNew York

Los Angeles

When projection is important

Los Angeles

Projection: MercatorDistance: 3,124.67 miles

Projection: Albers Equal AreaDistance: 2,455.03 miles

Actual distance: 2,451 miles

Los Angeles


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When projection is not important

Many business, policy, and management applications

On large-scale maps Error is negligible


MAP COORDINATESNow here, know where, or nowhere?

Latitude and longitude° longitude (prime meridian)0

° latitude (equator)

0


Latitude and longitude

Pittsburgh, PA USA

-80

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Coordinates


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°


Lat/long coordinates

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

Length of the equator is 24,901.55 mi


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GCS example (census tracts)


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Rectangular coordinates

UTM (universal transverse Mercator)

US military

State plane

Local US governments


UTM coordinates example

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


State plane coordinates 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)


State plane zones 125 zones

At least one for each state Cannot join zones to make larger regions Follow state and county boundaries

Each zone has its own tuned projection Lambert conformal projection for zones with

eastwest orientation Transverse Mercator projection for zones with

northsouth orientation


State plane zones


State plane coordinates example

State plane NAD 1983, Pennsylvania South, Feet


X,Y coordinate tips Always assign coordinates according to the

agencyUS CensusGeographic coordinate system (GCS)

City of PittsburghState plane coordinate system


X,Y coordinate examples

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US CensusGeographic coordinates (GCS)Block groups

City of PittsburghState plane coordinatesSidewalks


Map document tip 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 Known as .prj files


Map document tip Example: Sidewalks added first (state plane), but block

groups match even though they are in geographic coordinate system (GCS) projection.


US CENSUS GEOGRAPHIC FILES

Lecture 5

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Census TIGER/Line files

http://www.census.gov/geo/www/tiger/ Topologically Integrated

Geographic Encoding and Referencing files

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


http://www.census.gov/geo/www/tiger/

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Example census geographies


TIGER census tracts 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


State tracts (2010)


County tracts (2000 and 2010)


City tracts (2000 and 2010)


City block groups (2000 and 2010) 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)


Census blocks 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.)


US CENSUS DATA FILESLecture 5

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Decennial census data Years 2000 and 2010

Summary File 1 (SF 1) Short form, entire population Population Age Sex Race Families Households Housing units

Tracts, block groups, blocks


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Decennial census data Year 2000 Summary File 3 (SF 3)

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) 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


ACS Data 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 essentials51


ACS 1-year estimates 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


ACS 3-year estimates 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


ACS 5-year estimates 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. 54


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Downloading block group data http://www.census.gov/acs/www/data_documentation/summa

ry_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


http://www.census.gov/acs/www/data_documentation/summary_file/



http://www2.census.gov/acs2010_5yr/summaryfile/

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Other census data

Economic census

Population estimates

Annual economic surveys

DataFerret

http://dataferrett.census.gov/


http://dataferrett.census.gov/

GEOSPATIAL DATA SOURCESLecture 5

Spatial data infrastructure 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


Spatial data packaging Metadata

Documentation enabling intelligent use and interpretation

Data contents Provided by geographic area (political, statistical,

tile) or seamlessly (with extraction by area) Quality of geographic features

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


Classification of map layers

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

Map layers Physical features:

Earth’s surface and subsurface

Environmental features: atmosphere, climate, and weather

Living thing populations: people, animals, plants, and microbes

Organizational features: political, legal, administrative, and ecosystem


National Map orthoimagery 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


http://nationalmap.gov/viewer.html

http://nationalmap.gov/viewer.html

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


National Elevation Data (NED)

http://ned.usgs.gov/



http://nationalmap.gov/viewer.html/ Natural and man-made

surface features Collected from satellites in

1992, 2001, and 2006


Land cover

http://seamless.usgs.gov/

http://seamless.usgs.gov/

http://nhd.usgs.gov/ Water bodies, lines,

and points Identifies segments

(reaches) with network coding (flow and direction)


National Hydrography Dataset

http://nhd.usgs.gov/

http://nhd.usgs.gov/

USGS national water datasets http://

waterdata.usgs.gov/nwis/rt Streamflow conditions 5,000 stream gages with

telemetry transmits depth Program estimates flow rate


http://waterdata.usgs.gov/nwis/rt

http://waterdata.usgs.gov/nwis/rt

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Example geospatial sources Government websites (examples)

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


http://data.gov/

http://www.geoplatform.gov/home/



http://nationalatlas.gov/



http://data.gov/

http://nces.ed.gov/ccd/

http://nces.ed.gov/ccd/

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Example geospatial sources Commercial resources

(Esri, Google, engineering companies, etc.) Historic GIS websites

http://www.nhgis.org/ http://

www.aag.org/cs/projects_and_programs/historical_gis_clearinghouse

http://peoplemaps.esri.com/pittviewer/


http://www.nhgis.org/



http://www.aag.org/cs/projects_and_programs/historical_gis_clearinghouse





Summary

Models of the Earth Map coordinates Map projections US Census geographic files US Census data files Geospatial data sources


introduction to geographic information systems fall 2013 (inf 385t-28620) spatial reference systems,...

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