# global measures of spatial autocorrelation

Post on 13-Jan-2017

227 views

Category:

## Documents

Embed Size (px)

TRANSCRIPT

• Global Measures ofSpatial AutocorrelationBriggs Henan University 2010*

Briggs Henan University 2010

*

• Last TimeThe concept of spatial autocorrelation.Near things are more similar than distant thingsThe use of the weights matrix Wij to measure nearnessThe difficulty of measuring nearnessThis was a surprise!

This TimeMeasures of Spatial AutocorrelationJoin Count StatisticMorans IGearys CGetis-Ord G statistic

Briggs Henan University 2010*

Briggs Henan University 2010

*

• Global Measures and Local MeasuresBriggs Henan University 2010*An equivalent local measure can be calculated for most global measuresGlobal MeasuresA single value which applies to the entire data setThe same pattern or process occurs over the entire geographic areaAn average for the entire areaLocal MeasuresA value calculated for each observation unit Different patterns or processes may occur in different parts of the region A unique number for each location

Briggs Henan University 2010

*

• Briggs Henan University 2010*Join (or Joins or Joint) Count StatisticPolygons onlybinary (1,0) data onlyPolygon has or does not have a characteristicFor example, a candidate won or lost an electionBased on examining polygons which share a borderDo they have the same characteristic or not?Border same

on each sideBorder not the same

on each side Requires a contiguity matrix for polygons

Briggs Henan University 2010*

• Different numbers of BW, BB and WW joinsBriggs Henan University 2010*Join (or Joint or Joins) Count StatisticUses binary (1,0) dataShown here as B/W (black/white) Measures the number of borders (joins) of each type (1,1), (0,0), (1,0 or 0,1) relative to total number of bordersFor 6 x 6 matrix, border totals are:60 for Rook Case110 for Queen Case

Small number of BW joins (6 only for rook)Large proportion of BB and WW joins

Large number of BW joins Small number of BB and WW joins

Briggs Henan University 2010*

• Briggs Henan University 2010*Join Count: Test StatisticTest Statistic given by: Z= Observed - Expected SD of ExpectedExpected given by:Standard Deviation of Expected (standard error) given by:Where: k is the total number of joins (neighbors)pB is the expected proportion Black, if randompW is the expected proportion Whitem is calculated from k according to:Note: the formulae given here are for free (normality) sampling. Those for non-free (randomization) sampling are substantially more complex. See Wong and Lee 1st ed. p. 151 compared to p. 155. Se next slide for explanation.Expected = random pattern generated by tossing a coin in each cell.

Briggs Henan University 2010*

• A Note on Sampling Assumptions:applies to most tests for spatial autocorrelationTest results depend on the assumption made regarding the type of sampling:Free (or normality) sampling Analogous to sampling with replacementAfter a polygon is selected for a sample, it is returned to the population setThe same polygon can occur more than one time in a sample Non-free (or randomization) samplingAnalogous to sampling without replacementAfter a polygon is selected for a sample, it is not returned to the population setThe same polygon can occur only one time in a sample The formulae used to calculate the test statistic (particularly the standard error) are different for eachGenerally, the formulae are substantially more complex for free samplingunfortunately, it is also the more common situation!Assuming free sampling requires knowledge about larger trends from outside the region or access to additional information within the region in order to estimate parameters.

*Briggs Henan University 2010

Briggs Henan University 2010*

• Briggs Henan University 2010*total number of joins = 109 = sum of neighbors/2 in the sparse contiguity matrix= number of 1s/2 in the full contiguity matrix for US States (see slides from SA Concepts lecture)Gore/Bush Presidential Election 2000 Is there evidence of clustering by State?Use Join Count to answer this question!Many BB joins

Briggs Henan University 2010*

• Queens Case Sparse Contiguity Matrix for US StatesNcount is the number of neighbors for each stateEquals number of 1s in a row of full contiguity matrix Sum of Ncount is 218Number of common borders (joins) =

ncount / 2 = 109

N1, N2 FIPS codes for neighbors

*Briggs Henan University 2010

Briggs Henan University 2010

us49_q-Contig

NameFipsNcountN1N2N3N4N5N6N7N8

Alabama1428131247

Arizona4535849632

Arkansas56222848474029

California6343241

Connecticut93443625

Delaware103244234

District of Columbia1125124

Florida122131

Georgia135124537147

Idaho166324156493053

Illinois1752921185519

Indiana18426211739

Iowa196293117552746

Kansas2044029318

Kentucky21747291839545117

Louisiana22328485

Maine23133

Maryland2455110544211

Massachusetts255449365033

Michigan263183955

Minnesota27419554638

Mississippi284225147

Missouri298540172147201931

Montana30416563846

New Hampshire333252350

New Jersey343103642

New Mexico35548408449

New York365349425025

North Carolina37445134751

North Dakota383462730

Ohio3952621544218

Oklahoma4065354829208

Oregon4146321653

Pennsylvania426245410393634

Rhode Island442259

South Carolina4521337

South Dakota466562719313830

Tennessee47852813713512129

Texas4842253540

Utah4964835563216

Vermont503362533

Virginia516473724541121

Washington5324116

West Virginia5455121243942

Wisconsin55426171927

Wyoming56649163184630

us49_q-JC-%vote

Sparse Contiguity Matrix for US States -- obtained from Anselin's web site (see powerpoint for link)Bush/Gore 2000 Results

NameFipsNcountN1N2N3N4N5N6N7N8Winnerk*(k-1)

Alabama1428131247B04312

Arizona4535849632B05420

Arkansas56222848474029B06530

California6343241G1326

Connecticut93443625G1326

Delaware103244234G1326

District of Columbia1125124G1212

Florida122131B0212

Georgia135124537147B05420

Idaho166324156493053B06530

Illinois1752921185519G15420

Indiana18426211739B04312

Iowa196293117552746G16530

Kansas2044029318B04312

Kentucky21747291839545117B07642

Louisiana22328485B0326

Maine23133G1100

Maryland2455110544211G15420

Massachusetts255449365033B05420

Michigan263183955G1326

Minnesota27419554638G14312

Mississippi284225147B04312

Missouri298540172147201931B08756

Montana30416563846B04312

New Hampshire333252350B0326

New Jersey343103642G1326

New Mexico35548408449G15420

New York365349425025G15420

North Carolina37445134751B04312

North Dakota383462730B0326

Ohio3952621544218B05420

Oklahoma4065354829208B06530

Oregon4146321653G14312

Pennsylvania426245410393634G16530

Rhode Island442259G1212

South Carolina4521337B0212

South Dakota466562719313830B06530

Tennessee47852813713512129B08756

Texas4842253540B04312

Utah4964835563216B06530

Vermont503362533G1326

Virginia516473724541121B06530

Washington5324116G1212

West Virginia5455121243942B05420

Wisconsin55426171927G14312

Wyoming56649163184630B06530

Expected Values (number of joins)Totals19218880

(% of popular vote)from election resultsFormulae used for expected value calculation

Bush % (Pb)0.49885E(Jbb)=KPg^227.1248=Q55*B55^2k=total number of joins=218/2109

Gore % (Pg)0.50115E(Jgg)=KPb^227.3755=Q55*B56^2=880/2440

E(Jbg)=2KPbPg54.4997=2*Q55*B55*B56

Formulae used for Standard deviation calculation

StDev (bb)8.6673=SQRT(Q55*B55^2+2*Q56*B55^3-(Q55+2*Q56)*B55^4)

StDev (gg)8.7036=SQRT(Q55*B56^2+2*Q56*B56^3-(Q55+2*Q56)*B56^4)

StDev (bg)5.2203=SQRT(2*(Q55+Q56)*B55*B56-4*(Q55+2*Q56)*B55^2*B56^2)

Actual values (number of joins)z-scores= (actual - expected) / standev

from contiguity matrix aboveFormulae used for z-scores

BushActual(Jbb)603.7930=(F67-F55)/F60

GoreActual(Jgg)21-0.7325=(F68-F56)/F61

Bush/GoreActual(Jbg)28-5.0763=(F69-F57)/F62

(actually, I copied these from O&U, p 195)

The formulae here assume free (normality) sampling.

ExpectedStan DevActualZ-score

Bush % (Pb)0.49885Jbb27.1258.667603.7930

Gore % (Pg)0.50115Jgg27.3758.70421-0.7325

Jbg54.5005.22028-5.0763

us49_q-JC-%state

Sparse Contiguity Matrix for US States -- obtained from Anselin's web site (see powerpoint for link)Bush/Gore 2000 Results

NameFipsNcountN1N2N3N4N5N6N7N8Winnerk*(k-1)

Alabama1428131247AlabamaB04312

Arizona4535849632ArizonaB05420

Arkansas56222848474029ArkansasB06530

California6343241CaliforniaG1326

Connecticut93443625ConnecticutG1326

Delaware103244234DelawareG1326

District of Columbia1125124District of ColumbiaG1212

Florida122131FloridaB0212

Georgia135124537147GeorgiaB05420

Idaho166324156493053IdahoB06530

Illinois1752921185519IllinoisG15420

Indiana18426211739IndianaB04312

Iowa196293117552746IowaG16530

Kansas2044029318KansasB04312

Kentucky21747291839545117KentuckyB07642

Louisiana22328485LouisianaB0326

Maine23133MaineG1100

Maryland245