GIS Tutorial 1 Lecture 9 Spatial Analysis

Outline Proximity buffers Site suitability example Basic apportionment Advanced apportionment GIS TUTORIAL 1 - Basic Workbook 2

Lecture 9 Proximity buffers GIS TUTORIAL 1 - Basic Workbook

Proximity buffers Points Lines Circular buffers with user supplied radius Lines Looks like worm based on line feature GIS TUTORIAL 1 - Basic Workbook 4

Proximity buffers Polygons Extends polygons outward and rounds off corners Created by assigning a buffer distance around polygon GIS TUTORIAL 1 - Basic Workbook 5

Point buffer example Polluting company buffers Added schools Added population Data from U.S. Census and EPA website GIS TUTORIAL 1 - Basic Workbook 6

Point buffer example Crimes near schools 7 GIS TUTORIAL 1 - Basic Workbook 7

Line buffer example Businesses within .25 miles of a selected street GIS TUTORIAL 1 - Basic Workbook

Select features in buffer GIS TUTORIAL 1 - Basic Workbook 9

Spatial join to count Join business points to buffer polygon 10 GIS TUTORIAL 1 - Basic Workbook 10

Polygon buffer example Parcels within 150′ of selected property GIS TUTORIAL 1 - Basic Workbook

Select features in buffer GIS TUTORIAL 1 - Basic Workbook 12

Polygon buffer example River buffer to analyze environmental conditions, flooding, etc. GIS TUTORIAL 1 - Basic Workbook

Lecture 9 SITE Suitability GIS TUTORIAL 1 - Basic Workbook

Locate new police station Criteria Must be centrally located in each car beat (within a 0.33-mile radius buffer of car beat centroids) Must be in retail/commercial areas (within 0.10 mile of at least one retail business) Must be within 0.05 mile of major streets GIS TUTORIAL 1 - Basic Workbook

Starting map Lake Precinct of the Rochester, New York, Police Department Police car beats Retail business points Street centerlines GIS TUTORIAL 1 - Basic Workbook

Create car beat centroids XY centroids for police beats GIS TUTORIAL 1 - Basic Workbook

Buffer car beat centroids .33 mile buffer GIS TUTORIAL 1 - Basic Workbook 18

Buffer retail businesses 0.1 mile buffer GIS TUTORIAL 1 - Basic Workbook 19

Select major streets Select by attribute 20 GIS TUTORIAL 1 - Basic Workbook 20

Buffer major streets 0.05 mile buffer 21 GIS TUTORIAL 1 - Basic Workbook 21

Intersect buffers Can only intersect two at a time Car beat and businesses Streets GIS TUTORIAL 1 - Basic Workbook 22

Site suitability result Map showing possible sites for police station GIS TUTORIAL 1 - Basic Workbook 23

Lecture 9 Basic apportionment GIS TUTORIAL 1 - Basic Workbook

Apportionment example Population by voting district You want to know the population of a voting district but only have census tracts Voting districts and census tracts are not contiguous Approximate the population of voting using census tracts and blocks GIS TUTORIAL 1 - Basic Workbook 25

Population by voting district Start with census tracts GIS TUTORIAL 1 - Basic Workbook

Population by voting district Overlay voting districts (not contiguous with tracts) GIS TUTORIAL 1 - Basic Workbook 27

Population by voting district Better to use block centroids for population Smaller than tracts GIS TUTORIAL 1 - Basic Workbook

Spatially join centriods Join centroids to voting districts GIS TUTORIAL 1 - Basic Workbook 29

Other simple apportionments Population by Neighborhoods Zip Codes Historic sites Others? GIS TUTORIAL 1 - Basic Workbook

Census data to apportion Short form SF1 data (tract, block group, block) Population Age Race Housing Units Others? Long form SF3 data (tract and block group) Educational attainment Income Poverty status GIS TUTORIAL 1 - Basic Workbook 31

Advanced apportionment Lecture 9 Advanced apportionment GIS TUTORIAL 1 - Basic Workbook

Advanced Apportionment Chapter 9 example Police want to know the number of under-educated persons in their car beats Under-educated data is located SF3 tables, census tracts or block groups (not car beat polygons) GIS TUTORIAL 1 - Basic Workbook 33

Data to apportion Car beats Census tracts Beats and tracts Not contiguous GIS TUTORIAL 1 - Basic Workbook 34

Beats and tracts zoomed Tracts clearly cut across beats GIS TUTORIAL 1 - Basic Workbook 35

Tract attribute table Tracts contain undereducated data No high school degree GIS TUTORIAL 1 - Basic Workbook 36

Math of apportionment Simple census data (e.g. population) is not a problem Can use block centroids Problem Block centroids don’t contain undereducated population Tracts contain this information GIS TUTORIAL 1 - Basic Workbook 37

Math of apportionment Tract 360550002100 Car beats 261 and 251 Let’s take a look at one example. Above is a close-up of tract 360550002100, which is split between car beats 261 and 251. Tract 360550002100 has 205 people aged 25 or older with less than a high school education. For short, let’s call this the undereducated population. How can we divide those 205 undereducated persons between car beats 261 and 251? GIS TUTORIAL 1 - Basic Workbook 38

Math of apportionment One approach Assume that the target population is uniformly distributed across the tract You could split undereducated population up by the fraction of the area of the tract in each car beat What if, however, the tract has a cemetery, park, or other unoccupied areas? Then the apportionment could have sizable errors GIS TUTORIAL 1 - Basic Workbook 39

Math of apportionment A better approach Use a block-level, short-form census attribute as the basis of apportionment Assume that the long-form attribute of interest is uniformly distributed across the short-form population (accounts for unoccupied areas) One limitation of the block-level data is that the break points for age categories do not match those of the educational attainment data (persons 25 or older) The best that can be done with the block data is to tabulate persons aged 22 or older Close enough for approximation GIS TUTORIAL 1 - Basic Workbook 40

Math of apportionment Tract 360550002100 has 39 block centroids that span 2 beats Of the 26 blocks making up the tract, the 13 that lie in car beat 261 have 1,177 people aged 22 or older. The other 13 blocks in car beat 251 have 1,089 such people for a total of 2,266 for the tract. GIS TUTORIAL 1 - Basic Workbook 41

Math of apportionment Apportionment assumes that the fraction of undereducated people aged 25 or older is the same as that for the general population aged 22 or older This fraction, called the weight, is 1,177 ÷ 2,266 = 0.519. For the other car beat, the weight is 1,089 ÷ 2,266 = 0.481 Thus, we estimate the contribution of tract 36055002100 to car beat 261’s undereducated population to be (1,177 ÷ 2,266) × 205 = 106. For car beat 251, it is (1,089 ÷ 2,266) × 205 = 99 GIS TUTORIAL 1 - Basic Workbook 42

Math of apportionment Eventually, by apportioning all tracts, we can sum up the total undereducated population for car beats 261 and 251 GIS TUTORIAL 1 - Basic Workbook 43

Lecture 9 Background steps GIS TUTORIAL 1 - Basic Workbook

Background steps 1.) Download census data Download census block and tract polygons from the census Web sites for the county containing the administrative area polygons Download the short-form census data for blocks that are the basis of apportionment, in this case the population of age 22 and greater Download the long-form census attribute(s) at the tract level that you wish to apportion to the administrative area, in this case the population aged 25 or greater with less than high school education GIS TUTORIAL 1 - Basic Workbook 45

Background steps 2.) Create new tract layer That intersects administrative boundaries If a tract is only partially inside the administrative area, you must include the entire tract for apportionment to work correctly An example tract is the southerly-most tract in Tutorial9-3.mxd GIS TUTORIAL 1 - Basic Workbook 46

Background steps 3.) Prepare block centroids Create a new centroid point layer for blocks Clip the centroids with the new intersected tract layer Join census short-form data to the clipped block centroids This is the layer that is the basis for apportionment GIS TUTORIAL 1 - Basic Workbook 47

Background steps 4.) Sum the short-form census attributes in age categories to create Age22Plus in the clipped block centroids table This step is unique to this problem Also, this table has a new TractID attribute which concatenates FIPSSTCO & TRACT2000 to create an ID matching the Tracts map layer GIS TUTORIAL 1 - Basic Workbook 48

Background steps 5.) In the attribute table for block centroids, sum the field for persons aged 22 or older by TractID to create a new table, SumAge22Plus. This table provides the denominator for the weight used in apportionment GIS TUTORIAL 1 - Basic Workbook 49

Lecture 9 Apportionment steps GIS TUTORIAL 1 - Basic Workbook

Apportionment steps 1.)Intersect tracts and car beats to create new polygons that each have a tract ID and car beat number GIS TUTORIAL 1 - Basic Workbook 51

Apportionment steps 2.) Spatially join the new layer of tracts and car beats with the block centroids to assign all the tract attributes (including the attribute of interest: undereducated population) and car beat attributes to each block’s centroid GIS TUTORIAL 1 - Basic Workbook 52

Apportionment steps 2.) GIS TUTORIAL 1 - Basic Workbook 53

Apportionment steps 3.) Join SumAge22Plus to block centroids to make the apportionment weight denominator, total population aged 22 or older by tract, available to each block centroid GIS TUTORIAL 1 - Basic Workbook 54

Apportionment steps 3.) Export the join as a precaution 55 GIS TUTORIAL 1 - Basic Workbook 55

Apportionment steps 4.) For each block centroid, create new fields to store apportionment weight and apportioned undereducated population values, then calculate these values GIS TUTORIAL 1 - Basic Workbook 56

Apportionment steps 4.) Calculate values 57 GIS TUTORIAL 1 - Basic Workbook 57

Apportionment steps 5.) Sum the apportionment weights by tract as a check for accuracy (they should sum to 1.0 for each tract) GIS TUTORIAL 1 - Basic Workbook 58

Apportionment steps 5.) Each tract that is totally within car beats will have weights summing to 1. Those partially within car beats sum to less than 1 GIS TUTORIAL 1 - Basic Workbook 59

Apportionment steps 5.) Sum the undereducated population per car beat The extra row with no beat value is of no consequence, because it will not join to the car beats table in the next steps. GIS TUTORIAL 1 - Basic Workbook 60

Join apportionment results The last task is to join the table containing undereducated population by car beat to the car beats layer, then symbolize the data for map display GIS TUTORIAL 1 - Basic Workbook 61

Finished map There is some variation in the undereducated population in Lake Precinct car beats, but not a great deal. You can see that assault offenses tend to be in car beats or on the boundary of car beats with high undereducated populations. GIS TUTORIAL 1 - Basic Workbook 62

Summary Proximity buffers Site suitability example Basic apportionment Advanced apportionment GIS TUTORIAL 1 - Basic Workbook 63