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Overlay

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  1. Overlay A series of registered data layers ‘overlaying’ each other Arguably the most important GIS analysis function

  2. Overlay Derived from manual cartographic overlay using Mylar sheets (transparent plastic) that were physically overlaid on top of one another. -Ian McHarg in landscape architecture at University of PA

  3. Overlay • An overlay operation takes two or more data layers as input and results in an output data layer • Three types of overlay • Point in polygon • Line in polygon • Polygon on polygon • More complex in vector than in raster

  4. Overlay • Raster overlay • layers are registered • layers have same resolution

  5. 2 0 2 2 0 2 0 2 2 0 0 0 0 0 0 2 0 2 0 2 2 2 2 0 0 0 0 0 1 1 2 0 2 1 3 2 2 2 0 0 0 0 0 0 0 0 0 0 2 2 0 2 0 2 3 1 2 2 0 2 1 1 0 0 0 0 2 0 2 3 1 2 2 2 0 Overlay • Point in polygon: raster • Where are there trees in the urban area? ‘Point’ ‘Polygon’ Result + = 0 - not urban, no tree 1 - not urban, tree 2 - urban, no tree 3 - urban, tree 0 - not a tree 1 - tree 0 - not urban 2 - urban

  6. 2 0 0 2 0 2 0 0 0 0 0 2 2 0 0 2 0 0 1 0 0 2 3 1 0 2 0 0 0 0 0 0 0 2 2 0 3 1 0 2 0 1 0 0 0 2 3 1 2 0 Overlay • Point in polygon: raster • Where are there trees in the urban area? reclassify < 3 = 0 3 = 1 0 - not urban, no tree 1 - not urban, tree 2 - urban, no tree 3 - urban, tree 0 - not an urban tree 1 - urban tree

  7. 0 2 2 2 1 3 2 2 0 0 0 0 0 0 0 2 0 2 2 1 3 2 0 2 0 0 0 0 0 0 3 1 2 2 1 3 3 2 1 1 0 1 1 1 0 0 0 0 3 2 1 2 0 2 0 2 2 2 2 0 0 0 0 1 1 0 0 2 2 0 2 2 2 0 2 Overlay • Line in polygon: raster • Where are there roads in the urban area? ‘Line’ ‘Polygon’ Result + = 0 - not urban, no road 1 - not urban, road 2 - urban, no road 3 - urban, road 0 - not a road 1 - road 0 - not urban 2 - urban

  8. Overlay • Point in polygon: vector • Jacobsen plumbline algorithm Pass a line through the point and polygon if number of line/polygon intersections is: even - point is outside odd - point is inside

  9. Overlay • Point in polygon: vector • result must include attribute information Input Input Output 1 2 A A C + = C B B Point Table Poly Table Point Table ID Cover 1 Rural 2 Urban ID Tree A Elm B Maple C Elm ID Tree Cover A Elm Rural B Maple Rural C Elm Urban

  10. Overlay Input Input Output • Line in polygon: vector • result must include attribute information C C A A + = D B B 1 2 1 2 Point Table Poly Table Point Table ID Cover 1 Rural 2 Urban ID Street A Race B Race C Arch ID Street Cover A Race Rural B Race Urban C Arch Urban D Race Urban

  11. Overlay • Raster overlay • Boolean combination

  12. 0 0 1 1 0 0 0 0 1 0 0 1 0 1 0 0 1 0 0 1 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 1 0 1 1 1 1 1 1 0 0 1 1 0 0 1 0 0 1 1 1 1 1 1 Overlay • Raster overlay: polygon on polygon • where is agriculture on poor quality soil? Agr and poor soil Agriculture Poor soil quality

  13. Overlay • Raster overlay

  14. 1 0 1 0 0 1 0 0 0 0 0 1 0 1 1 0 0 0 0 1 1 1 1 1 1 1 0 1 0 1 2 2 2 2 2 1 2 0 0 2 2 2 2 2 2 1 0 2 0 2 Overlay • Raster overlay: polygon on polygon • where is agriculture on poor quality soil? Land cover Soil quality Output agriculture + poor = 2 forest + fair = 2 residential + good = 2 + = 0 - residential 1 - forest 2 - agriculture 0 - poor 1 - fair 2 - good

  15. 0 1 0 1 0 1 0 0 0 0 1 0 1 0 1 0 0 0 0 0 0 1 0 1 0 1 0 0 0 0 2 1 1 2 1 2 2 1 2 1 2 1 2 1 1 2 1 2 2 1 Overlay • Raster overlay: polygon on polygon • where is agriculture on poor quality soil? Land cover Agriculture reclassify 0 - residential 1 - forest 2 - agriculture 0 - not agriculture 1 - agriculture

  16. 0 1 1 0 0 1 0 1 1 0 0 1 0 1 0 1 0 1 0 1 1 0 0 1 1 0 1 0 0 1 0 2 0 2 1 0 0 1 0 1 0 2 0 2 1 0 1 0 0 1 Overlay • Raster overlay: polygon on polygon • where is agriculture on poor quality soil? Soil quality Poor soil quality reclassify 0 - poor 1 - fair 2 - good 0 - not poor 1 - poor

  17. Overlay 1 1 0 1 0 1 1 0 1 0 1 1 1 1 0 1 1 0 0 1 1 1 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 1 1 1 0 1 2 1 1 2 1 1 0 1 1 0 1 1 0 1 1 1 2 2 1 1 • Raster overlay: polygon on polygon • where is agriculture on poor quality soil? Output Agriculture Poor soil quality + = 0 - not agr., not poor 1 - agr., not poor or not agr, poor 2 - agr, poor 0 - not agriculture 1 - agriculture 0 - not poor 1 - poor

  18. Overlay 0 1 1 0 0 1 0 1 1 0 0 1 1 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 1 2 1 2 1 0 1 0 1 0 1 1 2 2 1 • Raster overlay: polygon on polygon • where is agriculture on poor quality soil? Output Final Output reclassify 0 - not agr. on poor soil 1 - agr. on poor soil 0 - not agr., not poor 1 - agr., not poor or not agr, poor 2 - agr, poor

  19. 0 0 0 0 0 0 1 0 1 1 1 0 1 1 0 0 1 1 1 2 1 1 1 0 1 1 0 1 1 2 2 2 0 2 1 1 1 2 3 1 0 1 1 1 0 3 2 1 2 4 2 4 2 2 1 3 2 1 0 1 2 1 3 2 2 4 4 2 2 2 3 1 2 0 2 Overlay Daily high temperature Daily high humidity • Raster overlay: map algebra Hot + Humid Index + = 0 - very low 1 - low 2 - medium 3 - high 4 - very high 0 - mild 1 - warm 2 - hot 0 - not humid 1 - semi humid 2 - very humid

  20. Overlay • Raster overlay: map algebra • can use addition, multiplication, etc. • can ‘weight’ certain data layers • can use any number of data layers • can’t use nominal data unless it used as inclusionary/exclusionary by reclassification! • Be careful with ordinal data - classification impacts the results of overlay! • Standardize interval/ratio data

  21. Overlay • Vector topologic overlay: Boolean operators Intersection (and) Union (or) Identity

  22. Overlay • Vector topologic overlay: intersection

  23. Overlay • Vector topologic overlay: intersection (and) Agriculture Good Quality Soil Build new topology A A B B ID Owner A Brown B Smith ID Quality A good B good Area of intersection New node

  24. Overlay Build new topology Output • Vector topologic overlay: intersection (or) A B ID Owner Quality A Brown good B Smith good Area of intersection New node

  25. Overlay • Vector topologic overlay: union Agriculture Good Quality Soil Build new topology A A B B ID Owner A Brown B Smith ID Quality A good B good Area of union New node

  26. Overlay Output Build new topology • Vector topologic overlay: union B C A D E ID Owner Quality A good B Brown good C Brown D Smith E Smith good Area of union New node

  27. Vector topologic overlay: identity Agriculture (input layer) Good Quality Soil (identity layer) Build new topology A A B B ID Owner A Brown B Smith ID Quality A good B good Area of identity New node

  28. Overlay • Vector topologic overlay: identity Output Build new topology A B C D ID Owner Quality A Brown good B Brown C Smith D Smith good Area of identity New node