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. Overlay Point in polygon: raster –Where are there trees in the urban area? 0 10 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 1 2 22 0 0 2 2 2 0 2 2 2 2 2 2 0 0 0 2 2 0 0 0 2 2 0 - not a tree 1 - tree ‘Point’‘Polygon’ 0 - not urban 2 - urban 0 - not urban, no tree 1 - not urban, tree 2 - urban, no tree 3 - urban, tree 2 32 0 1 2 2 2 0 2 2 2 2 2 2 0 0 1 2 2 0 0 0 3 3 += Result

6. Overlay Point in polygon: raster –Where are there trees in the urban area? 0 - not urban, no tree 1 - not urban, tree 2 - urban, no tree 3 - urban, tree 2 32 0 1 2 2 2 0 2 2 2 2 2 2 0 0 1 2 2 0 0 0 3 3 reclassify < 3 = 0 3 = 1 0 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 - not an urban tree 1 - urban tree

7. Overlay 0 11 0 0 1 1 1 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 2 22 0 0 2 2 2 0 2 2 2 2 2 2 0 0 0 2 2 0 0 0 2 2 0 - not a road 1 - road ‘Line’‘Polygon’ 0 - not urban 2 - urban 0 - not urban, no road 1 - not urban, road 2 - urban, no road 3 - urban, road 2 33 0 0 3 3 3 1 2 2 3 2 2 2 1 0 0 2 2 1 0 0 2 2 += Result Line in polygon: raster –Where are there roads in the urban area?

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 A B C ID Tree A Elm B Maple C Elm Point Table ID Tree Cover A Elm Rural B Maple Rural C Elm Urban Point Table ID Cover 1 Rural 2 Urban Poly Table 12 + A B C= Input Output

10. Overlay Line in polygon: vector –result must include attribute information A B C ID Street A Race B Race C Arch Point Table ID Street Cover A Race Rural B Race Urban C Arch Urban D Race Urban Point Table ID Cover 1 Rural 2 Urban Poly Table 12 += Input Output A B C 12 D

11. Overlay Raster overlay –Boolean combination

12. Overlay Raster overlay: polygon on polygon –where is agriculture on poor quality soil? Agriculture 0 00 1 1 0 0 1 1 0 0 0 1 1 0 0 0 0 1 1 0 0 0 1 1 Poor soil quality 1 00 0 0 1 1 0 0 1 1 0 0 0 1 0 1 0 1 1 0 1 1 1 1 Agr and poor soil 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 1

13. Overlay Raster overlay

14. Overlay Raster overlay: polygon on polygon –where is agriculture on poor quality soil? 1 11 2 2 1 1 2 2 1 1 1 2 2 1 0 0 0 2 2 0 0 0 2 2 0 11 2 2 0 0 2 2 0 0 1 1 1 0 1 0 1 0 0 1 0 0 0 0 0 - residential 1 - forest 2 - agriculture Land coverSoil quality 0 - poor 1 - fair 2 - good += Output agriculture + poor = 2 forest + fair = 2 residential + good = 2

15. Overlay Raster overlay: polygon on polygon –where is agriculture on poor quality soil? 1 11 2 2 1 1 2 2 1 1 1 2 2 1 0 0 0 2 2 0 0 0 2 2 0 - residential 1 - forest 2 - agriculture Land coverAgriculture 0 00 1 1 0 0 1 1 0 0 0 1 1 0 0 0 0 1 1 0 0 0 1 1 reclassify 0 - not agriculture 1 - agriculture

16. Overlay Raster overlay: polygon on polygon –where is agriculture on poor quality soil? 0 - not poor 1 - poor Soil qualityPoor soil quality 1 00 0 0 1 1 0 0 1 1 0 0 0 1 0 1 0 1 1 0 1 1 1 1 reclassify 0 11 2 2 0 0 2 2 0 0 1 1 1 0 1 0 1 0 0 1 0 0 0 0 0 - poor 1 - fair 2 - good

17. Overlay Raster overlay: polygon on polygon –where is agriculture on poor quality soil? += Output Agriculture 0 00 1 1 0 0 1 1 0 0 0 1 1 0 0 0 0 1 1 0 0 0 1 1 0 - not agriculture 1 - agriculture 0 - not poor 1 - poor Poor soil quality 1 00 0 0 1 1 0 0 1 1 0 0 0 1 0 1 0 1 1 0 1 1 1 1 1 00 1 1 1 1 1 1 1 1 0 1 1 1 0 1 0 2 2 0 1 1 2 2 0 - not agr., not poor 1 - agr., not poor or not agr, poor 2 - agr, poor

18. Overlay Raster overlay: polygon on polygon –where is agriculture on poor quality soil? Output 1 00 1 1 1 1 1 1 1 1 0 1 1 1 0 1 0 2 2 0 1 1 2 2 0 - not agr., not poor 1 - agr., not poor or not agr, poor 2 - agr, poor Final Output 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 1 0 - not agr. on poor soil 1 - agr. on poor soil reclassify

19. Overlay Raster overlay: map algebra += Hot + Humid Index Daily high temperature 0 10 1 1 0 1 2 2 0 1 2 2 2 1 1 0 1 1 2 1 1 1 2 2 0 - mild 1 - warm 2 - hot 0 - not humid 1 - semi humid 2 - very humid Daily high humidity 0 12 2 2 0 1 2 2 1 0 1 2 2 0 0 1 1 0 0 0 0 0 1 1 0 22 3 3 0 2 4 4 1 1 3 4 4 1 1 1 2 1 2 1 1 1 3 3 0 - very low 1 - low 2 - medium 3 - high 4 - very high

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 A B A Good Quality Soil ID Owner A Brown B Smith ID Quality A good B good B Area of intersection New node Build new topology

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

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

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

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

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