1. ©2005 Austin Troy Lecture 9: Introduction to GIS 1.Vector Geoprocessing Lecture by Austin Troy, University of Vermont

2. ©2005 Austin Troy Geoprocessing in Context Last time we learned how to select features based on locations and to join attributes based on location This time we will learn about how to actually change the geometry of vector features to more efficiently answer our questions We have three general classes of tools at our disposal: –Breaking features into smaller features (usually polygons) –Aggregating features into larger features (ditto) –Creating new polygon features through buffering First I’ll introduce the tools, then I’ll show how they can be integrated to dramatically increase your GIS toolbox. Introduction to GIS

3. ©2005 Austin Troy Tools: Geoprocessing In ArcGIS, the GP wizard allows you to dissolve, clip, merge, intersect, union Introduction to GIS

4. ©2005 Austin Troy Purpose of Geoprocessing Tools for breaking down the size of map features: –Union, Intersect, Clip Tools for increasing the size of map features: –dissolve and merge (indirectly) Arc/Info and Arc Toolbox include various other geoprocessing overlay operations, such as Update and Dissolve Regions Introduction to GIS

5. ©2005 Austin Troy Union vs. Intersection Remember Set Theory from Geometry? Union is the union of two overlapping set of features and intersection is the intersection What SQL query operators do these correspond with? Introduction to GIS Layer 1 +Layer 2 Intersect: Layer 1 + Layer 2 Union: “1 AND 2” “1 OR 2”

6. ©2005 Austin Troy Union Combines features of two theme Each theme is treated the same Goes to extent of largest theme Keeps all line work, creates new polygons Breaks down features into smaller minimum mapping units Can use selected features option too Keeps all attributes Introduction to GIS

7. ©2005 Austin Troy Intersect Yields polygons representing areas that are common to both layers Preserves line work within common extent Usually creates many new, smaller polygons Preserves all attributes from both Introduction to GIS

8. ©2005 Austin Troy Union vs. Intersection: Example Here’s an example. Say we have deer wintering areas in one layer and conserved lands in another. Introduction to GIS

9. ©2005 Austin Troy Union vs. Intersection: Example Union gives us land that is EITHER conserved OR that is a deer wintering areas Introduction to GIS

10. ©2005 Austin Troy Union vs. Intersection: Example Intersect gives us land that is BOTH, and preserves all polygon boundaries within that common extent Introduction to GIS

11. ©2005 Austin Troy Clip This uses one theme to “clip,” or serve as the outer boundary of another theme EG, clip the CA highways theme to include only highways for Merced County Can clip by a single selected feature in a theme Introduction to GIS This is another way to break down features into smaller units

12. ©2005 Austin Troy Clipping highways for Merced Introduction to GIS Note that the “use selected features only” option was used

13. ©2005 Austin Troy Dissolve This is a single layer operation Tool for aggregating polygons—making them bigger. Allows you to join together into one polygon multiple adjacent polygons with the same value for a specified attribute Introduction to GIS

14. ©2005 Austin Troy Dissolve Example: Zip codes are nested within counties. The zip code layer has county as an attribute. Therefore I can dissolve zip codes (small) into counties (large) Introduction to GIS

15. ©2005 Austin Troy Dissolve I choose to dissolve based on the County field Introduction to GIS Then I must choose how to summarize the resulting field values, since, for a given field, many values are being merged into one field. For instance, I could sum population for each county

16. ©2005 Austin Troy Dissolve Now we have created a county map, and for each county we have as an attribute the sum of population of the constituent zip codes Introduction to GIS

17. ©2005 Austin Troy Merge Allows you to “join” two adjacent or non- adjacent themes into the same layer Like “tiling” Best when attributes match Introduction to GIS

18. ©2005 Austin Troy Merge Often when you merge you will want to follow up by dissolving. This is because artificial polygon boundaries were created at the borders by the act of splitting the data up into tiles; merging brings the tiles back together and dissolving joins together polygons that were split up in the process Introduction to GIS

19. ©2005 Austin Troy Tools: Buffering Buffering is when you draw a polygon around a feature (point, line or polygon); Here we’re buffering a stream Introduction to GIS

20. ©2005 Austin Troy Tools: Buffering ArcGIS has a “buffer wizard” Allows for variable width based on attribute Allow multiple buffers Introduction to GIS

21. ©2005 Austin Troy Tools:Variable Width Buffering Where width of buffer varies with an attribute Often we must recalculate that attribute to make the buffer width meaningful Example from lab: a buffer based on traffic volume Introduction to GIS

22. ©2005 Austin Troy Combining Buffering and Geoprocessing Last time we learned how to select a feature in layer A if it was within a distance X of a feature in layer B Buffer offers a different approach for doing the same. By building a buffer of a given distance around a feature, we can then do overlay analysis and select features from another layer that are inside or that intersect the buffer The difference is that with selecting by location, we can only do 2 layers at a time. When we combine buffering with geoprocessing, we can ask questions across unlimited numbers of layers. Introduction to GIS

23. ©2005 Austin Troy Combining Buffering and Geoprocessing: Example From Lab 6: say we made fixed buffers around deer wintering areas and water bodies, and a variable buffer around roads, based on traffic: Introduction to GIS ( note the lab is a bit different ):

24. ©2005 Austin Troy Combining Buffering and Geoprocessing: Example Then we could, for instance, find areas that are near deer wintering areas and water bodies but far from traffic: First we would intersect the deer wintering and water body buffers, yielding areas that are near both those. Then we would union the resulting with the traffic buffer and run an attribute query to determine which areas meet the criteria Introduction to GIS

25. ©2005 Austin Troy Combining Buffering and Geoprocessing: Example The intersection of deer wintering buffers and water buffers would be the area in the red Introduction to GIS

26. ©2005 Austin Troy Combining Buffering and Geoprocessing: Example The union of that intersection with the traffic buffer: Introduction to GIS

27. ©2005 Austin Troy Combining Buffering and Geoprocessing: Example Now we can query for polygons that were created from the intersection (because they met two of the criteria) and for areas that are not within a traffic buffer Introduction to GIS

28. ©2005 Austin Troy Combining Buffering and Geoprocessing: Example We can then create a layer from that—Note that we have created entirely new polygon boundaries and geometry by cutting and splicing these buffers together. Introduction to GIS

29. ©2005 Austin Troy Buffering and selecting Of course, we can also use buffers to assist with “select by location” but we could do that using the select by location distance function more easily Introduction to GIS

30. ©2005 Austin Troy Combining Union and Dissolve In many cases you will want to join two layers with different polygons so that you can attach an attribute from one coverage to the second coverage Use Union, to “squash” them together. Then dissolve the theme back to the unit of geography you care about, based on that unit’s unique ID. When you dissolve, you can summarize the new attribute by mean, max, min, stdv, etc. This is how we take attribute info in polygon layer A and assign it to differently shaped polygons in layer B Introduction to GIS

31. ©2005 Austin Troy Combining Union and Dissolve Confused? Here’s an example: Say you have a hill slope polygon theme with small polygons and a vegetation theme with larger polygons and you are using the vegetation stands as your management units; you want to get an average slope value for every vegetation polygon without having thousands of slope polygons. Do a union or intersect of the two Now dissolve polygons based on vegetation polygon ID; all those new polygon fragments that formerly belonged to the same vegetation fragment will be reunited. Choose “average of slope” as the option to summarize by when you merge; it averages all the slope values for each vegetation polygon, based on area. Introduction to GIS