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GIS Tutorial 1

GIS Tutorial 1. Lecture 9 Spatial Analysis. Outline. Proximity buffers Site suitability example Basic apportionment Advanced apportionment. 2. Lecture 9. Proximity buffers. Proximity buffers. Points Circular buffers with user supplied radius Lines

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GIS Tutorial 1

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  1. GIS Tutorial 1 Lecture 9 Spatial Analysis

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

  3. Lecture 9 Proximity buffers GIS TUTORIAL 1 - Basic Workbook

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

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

  6. Point buffer example Polluting company buffers • Added schools • Added population GIS TUTORIAL 1 - Basic Workbook 6

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

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

  9. Select features in buffer GIS TUTORIAL 1 - Basic Workbook 9

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

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

  12. Select features in buffer GIS TUTORIAL 1 - Basic Workbook 12

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

  14. Lecture 9 SITE Suitability GIS TUTORIAL 1 - Basic Workbook

  15. 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

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

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

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

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

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

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

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

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

  24. Lecture 9 Basic apportionment GIS TUTORIAL 1 - Basic Workbook

  25. 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

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

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

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

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

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

  31. 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 • Others? GIS TUTORIAL 1 - Basic Workbook 31

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

  33. 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

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

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

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

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

  38. Math of apportionment Tract 360550002100 Car beats 261 and 251 GIS TUTORIAL 1 - Basic Workbook 38

  39. 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

  40. 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

  41. Math of apportionment 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. Tract 360550002100 has 39 block centroids that span 2 beats GIS TUTORIAL 1 - Basic Workbook 41

  42. 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

  43. 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

  44. Lecture 9 Background steps GIS TUTORIAL 1 - Basic Workbook

  45. 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

  46. 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

  47. 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

  48. 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

  49. 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

  50. Lecture 9 Apportionment steps GIS TUTORIAL 1 - Basic Workbook

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