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GIS Lecture 9 Spatial Analysis

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GIS Lecture 9 Spatial Analysis. Outline. Proximity Buffers Spatial Joins on Buffers Visual Basic Scripts Apportionment. Proximity Buffers. Proximity. Buffers Created Points Lines Polygons. Points. Buffer created by assigning a buffer distance around points.

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GIS Lecture 9

Spatial Analysis

  • Proximity Buffers
  • Spatial Joins on Buffers
  • Visual Basic Scripts
  • Apportionment

Buffers Created

  • Points
  • Lines
  • Polygons

Buffer created by assigning a buffer distance around points

green spaces near schools
Green Spaces Near Schools

- Polygon buffer created ¼ and ½ mile around schools

businesses near convention center
Businesses near Convention Center
  • Businesses that are with ¼ mile of Convention Center
campus safety
Campus Safety

Polygon buffer 20’ around lights in a parking lot


Buffer created by assigning a buffer distance around lines


Line Buffer Example

Access-to-Work Study (Pittsburgh Foundation)

- Polygon buffer created around PAT Bus Routes

- Shows 15 minute ride times


Line Buffer Example

Another buffer shows 30 minute ride times


Line Buffer Example

…45 minutes


Line Buffer Example

…60 minutes


Line Buffer Example

Street Study

- Polygon buffer created ¼ mile around selected streets


Buffer created by assigning a buffer distance around polygons

spatial joins on buffers
Spatial Joins on Buffers
  • Spatially join Toxic Release sties to buffers
  • Count appears in new buffer
visual basic scripts
Visual Basic Scripts
  • Adding Area and Perimeter to Polygons
  • Finding Polygon Centroids
  • Exporting to Google Earth
area and perimeter vb script
Area and Perimeter VB Script

Advanced calculations for finding area, perimeter, and length of features

area and perimeter vb script23
Area and Perimeter VB Script
  • Add field in shapefile (e.g. area)
  • Use calculator function and Visual Basic Script to calculate polygon areas
area and perimeter vb script24
Area and Perimeter VB Script

Result is the area of each polygon feature

polygon centroids
Polygon Centroids
  • Advanced calculations for finding polygon centroids
  • Added as an XY Data Layer
polygon centroids27
Polygon Centroids

Export attributes as table

Add as XY Data…

export to google earth script
Export to Google Earth Script
  • 1. Go to and download the ArcView script for exporting.
  • 2. Unzip the files to a new folder.
  • 3. Run the install file called ExporttoKML_INSTALL.bat.
  • 4. Launch ArcView.
  • 5. Click Tools, Extensions..., and the new Export to KML extension. A dialog box will appear with a little globe. If it doesn't appear, right click on the toolbar and load it from there.
  • 6. Click the globe and follow the dialog box prompts to export to Google Earth.

Apportionment Example

  • You want to know the population of a ZIP code but only have census tracts
  • Approximate the population of zip codes using Census Tracts
population apportionment
Population Apportionment
  • Begin with census tract population
population apportionment34
Population Apportionment
  • Overlay zip codes which are non-coterminous
population apportionment35
Population Apportionment
  • Use census tracts to estimate the population in each zipcode
  • Or use census blocks for better estimates
other examples of apportionment
Other examples of apportionment

Population by voting district

other census data to apportion
Other census data to apportion…
  • SF1 Census Data
    • Population (tract, block group, and block)
    • Age (tract, block group, and block)
    • Race (tract, block group, and block)
    • Housing Units (tract, block group, and block)
  • SF3 Census Data
    • Educational Attainment (tract and block group only)
    • Income (tract and block group only)
    • Poverty Status (tract and block group only)
    • Others?
advanced apportionment tutorial example
Advanced Apportionment:Tutorial Example
  • Police want to know the number of under-educated persons (over Age22) in their car beats
  • Under-educated data is located in Census tracts (not car beat polygons)

Apportion Data for Non-CoterminousPolygons

Apportioning (makes approximate splits) of each tract’s data to two or more car beats.

approach to apportionment
Approach to Apportionment

Better to use census block centroid data

  • Areas are smaller than Census Tracts (better population estimates)
  • Each centroid has census data attached to it
  • Problem:Centroids DO NOThave the under-educateddata, census tracts do
the math of apportionment
The Math of Apportionment

Zoomed view of 2 car beats and one tract

  • Beat 261 and 251
  • Tract 360550002100
the math of apportionment42

13 block centroids

  • Lie in beat 261
  • Pop. >22=1,177
  • 13 block centroids
  • Lie in beat 251
  • Pop. >22=1,089
The Math of Apportionment

Tract 360550002100

  • has 205 persons aged 25 or older with less than a HS education
  • 26 block centroids span 2 beats

Total Population=2,266

the math of apportionment43
The Math of Apportionment

Apportionment assumes that the fraction of under educated persons 25 or older is the same as that for the general population aged 25 or older:

  • Beat 261: 1,177/2,266 = 0.519
  • Beat 251: 1,089/2,266 = 0.481
the math of apportionment44
The Math of Apportionment
  • 205 is the number of under-educated people in tract 36055002100
  • Thus we estimate the contribution of tract 36055002100 to car beat 261’s under-educated population to be (1,177/2,266)x205 = 106. For car beat 251 it is (1,089/2,266)x205 = 99
  • To calculate this in GIS, we need to performintersects and joins…
apportionment steps
Apportionment Steps

Block Centroids

  • Add two fields: TRACTID and SumAge22
    • TRACTID is a the census tract ID numbers (for later joins to census tracts and summaries)
    • SumAge22 is the summary of population Age22+ (calculating multiple age columns)
apportionment steps46
Apportionment Steps

From the block centroids, create a new summary table counting the number of persons Age22+ for each census tract

apportionment steps47
Apportionment Steps
  • Create a new layer intersecting car beats and census tracts
  • Fields will include values from both tables
apportionment steps48
Apportionment Steps
  • Spatially join the new intersecting layer of car beats and census tracts (polygons) to block centroids (points)
  • New points will have beat and census tract data
apportionment steps49
Apportionment Steps

Check Apportionment Weights

  • Create apportionment weight denominator
    • Join the summary table of Age22 or greater to the newly created points
    • The result is the summary of Age22 or greater population is now on block centroid points
apportionment steps50
Apportionment Steps
  • Compute Apportionment Weights
    • Create new block centroid field called Weight
apportionment steps51
Apportionment Steps
  • Create new block centroid field called UnderEducated
apportionment steps52
Apportionment Steps
  • Sum Weights by Tract

Values should all

be 1.00

apportionment steps53
Apportionment Steps
  • Sum Under Educated by Car Beat
  • Proximity Buffers




  • Spatial Joins on Buffers
  • Visual Basic Scripts


Point Centroids

  • Apportionment