1. ©2007 Austin Troy Lecture 8: Introduction to GIS 1.Multi-layer vector query operations in Arc GIS 2.Vector Spatial Joining Lecture by Austin Troy, University of Vermont

2. ©2007 Austin Troy Introduction to GIS 1. Multi-layer vector queries in Arc GIS

3. ©2007 Austin Troy Introduction to GIS Let’s say we want get information about all the houses in four sample neighborhoods and see which ones overlay fire hazard zones Selecting By Location

4. ©2007 Austin Troy Introduction to GIS Now with “sample houses” active, we click select by theme and tell it to choose features that intersect the features of fire hazard zone Layer to be selected Selection Method Selection rule Selection overlay theme Selecting By Location

5. ©2007 Austin Troy Introduction to GIS Those that overlay a hazard zone are selected selected Not selected Selecting By Location

6. ©2007 Austin Troy Introduction to GIS …Zooming in to one of those neighborhoods Selecting By Location

7. ©2007 Austin Troy Introduction to GIS Now run statistics on the selection; 1955 houses overlay fire zones; mean price is $467,551! Selecting By Location

8. ©2007 Austin Troy Introduction to GIS Invert selection; non fire zone houses are worth less on average!Only $246,752 Selecting By Location

9. ©2007 Austin Troy Introduction to GIS Select houses within 1 mile of a Starbucks Selecting By Location:Distance

10. ©2007 Austin Troy Introduction to GIS This time we use a different selection method with different parameters Selecting By Location :Distance Note how we can specify the distance for selection

11. ©2007 Austin Troy Introduction to GIS Results in the following selection Selecting By Location :Distance

12. ©2007 Austin Troy Introduction to GIS Zooming into a neighborhood… Selecting By Location :Distance

13. ©2007 Austin Troy Introduction to GIS Now if we run statistics on price again… Selecting By Location :Distance Those within a mile of a Starbucks have a mean value of $504,972 Those not within a mile of a Starbucks have a mean value of $273,866! By the way, these are real data, I’m not making this up!!

14. ©2007 Austin Troy Introduction to GIS For that same selection we could get statistics on a different variable—here we’ll look at lot size Selecting By Location :Distance Those within a mile of a Starbucks have a mean size of 8776 square feet Those not within a mile of a Starbucks have a mean lot size of 10,024 sq feet. Why might that be?

15. ©2007 Austin Troy Introduction to GIS Linear feature: selecting houses in a neighborhood within a mile of a highway Selecting By Location :Distance Note that these smaller roads are in a different layer

16. ©2007 Austin Troy Introduction to GIS Selecting based on existing selection Find homes within 500 meters of Valley Blvd. (let’s say there’s going to be a parade and the city needs to inform all those homeowners near that street). Query Hwyname = “Valley Blvd” Selecting By Location :Distance

17. ©2007 Austin Troy Introduction to GIS Once that feature is selected we can do a “select by location” operation Selecting By Location :Distance Notice that this time we check “Use selected features”

18. ©2007 Austin Troy Introduction to GIS Thus we end up only selecting those houses within 500 m of Valley Blvd, and none within 500 m of other roads Selecting By Location :Distance

19. ©2007 Austin Troy Introduction to GIS Spatial overlap problem: whole polygon will be selected even if only a small part is coincident, assuming we are using the default selection overlay method, “intersect.” Selecting By Location: Polygons However, there are many other methods we can choose from that will change the number of polygons selected.

20. ©2007 Austin Troy Introduction to GIS Example: let’s select any census tract that intersects even slightly with a fire zone; here’s the pre-selection map Selecting By Location: Polygons

21. ©2007 Austin Troy Introduction to GIS Using the “intersect” overlay method we get this Selecting By Location: Polygons

22. ©2007 Austin Troy Introduction to GIS Using “that are completely within” method, we get no selected feature. But, with “have their center in” we get Selecting By Location: Polygons

23. ©2007 Austin Troy Likewise, if we select Merced County in the counties layer, activate “highways” in the TOC, and then select by theme, we will only choose those road segments that intersect that county Introduction to GIS Selecting By Location on Selections

24. ©2007 Austin Troy Note that the resulting selection was made using the “intersect method.” If the “completely within” method is used, a different set of lines will be selected Introduction to GIS Selecting By Location on Selections

25. ©2007 Austin Troy Left: intersect method Right: completely within method Introduction to GIS Selecting By Location on Selections

26. ©2007 Austin Troy Once a selection has been done using “select by location” you can do all the same things you would do with a normal single-layer selection: –Make a new layer from the selection –Do statistics on it –Make a new field in that layer (e.g.a field called “Parade”, where “yes” means the house is within 500m of the parade route). –Calculate or recalculate a field for a selection Introduction to GIS What can be done with multi-layer selections?

27. ©2007 Austin Troy Introduction to GIS 2. Vector Spatial Joining — assigning attributes by location

28. ©2007 Austin Troy Spatial Join Introduction to GIS Assigns attribute data from features in one layer to spatially coincident features in another Can assign polygon data to a point that overlays Can assign point to point and point to line distances between two layers Simply adds attributes to the DBF table

29. ©2007 Austin Troy Spatial Join Introduction to GIS We access Spatial Join by right clicking on the “to” layer and clicking Joins and Relates>>join We then specify that we want to join by location and choose which layer we are joining from

30. ©2007 Austin Troy Spatial Join Introduction to GIS In this case we are going to join tracts to the houses from our sample neighborhoods. Each house inherits all the attributes of the tract in which it falls. Note that this creates a new layer

31. ©2007 Austin Troy Spatial Join Introduction to GIS Plot of houses graduated by percent unemployment of the tract to which they belong

32. ©2007 Austin Troy Spatial Join:Distance Introduction to GIS We can also do spatial joins based on distance. Whenever we join a point or line layer to another point or line layer, for each feature in the TO layer it gives us the attributes of the nearest feature in the FROM layer PLUS the distance between those features in whatever map units we specify

33. ©2007 Austin Troy Spatial Join:Distance Introduction to GIS Use Spatial Join to assign as attribute to our house point layer the name of the nearest major road.

34. ©2007 Austin Troy Spatial Join:Distance Introduction to GIS Two options: choose to numerically summarize for each point the values of the lines intersecting it, or assign all attributes from the nearest line. My FROM layer

35. ©2007 Austin Troy Spatial Join:Distance Introduction to GIS Now name of nearest highway is an attribute for each housing point

36. ©2007 Austin Troy Spatial Join:Distance Introduction to GIS Distance from each point to the nearest road feature was also recorded under the attribute “Distance.”

37. ©2007 Austin Troy Spatial Join:Distance Introduction to GIS We can also do a join to get the distance from a series of points in one layer to a series of points in another: here is distance of houses to nearest Starbucks

38. ©2007 Austin Troy Spatial Join:Polygons Introduction to GIS What about polygons? Problem: a polygon is layer A may overlay several polygons in layer B, so whose attributes to you give it? Layer A Layer B

39. ©2007 Austin Troy Spatial Join:Polygons Introduction to GIS Answer: we can do spatial join and summarize (by average, for instance) each polygon in layer A the values of all the overlapping polygons in layer B. Example: Marketing study; have a census tract layer with all sorts of demographic info (population, race, etc) and a zip code layer with no demographic info attached to it. Client needs map showing median age and percent Hispanic by zip code.

40. ©2007 Austin Troy Spatial Join:Polygons Introduction to GIS Unfortunately, the tract boundaries and zip code boundaries do not match up in the slightest. Note that tracts are not nested within zip codes—they cut across

41. ©2007 Austin Troy Spatial Join:Polygons Introduction to GIS Do a spatial join of two polygon layers and choose the “summarize” option (the first radio button). Choose a statistic by which to summarize values

42. ©2007 Austin Troy Spatial Join:Polygons Introduction to GIS Plot of median age

43. ©2007 Austin Troy Vector Limitations Introduction to GIS Multi-layer vector polygon analysis is limited in that generally polygons in different layers are of different sizes and shapes: irregular minimum mapping unit Next week we will discuss how vector geoprocessing can be used to help overcome this to a certain extent Geoprocessing includes methods by which vector features are broken down into smaller features, or aggregated into larger features.