Geog. 377: Introduction to GIS - Lecture 21 Overheads 1 7. Spatial Overlay 7.1 Map Algebra (raster) 7.2 Polygon Overlay (vector) Topics Lecture 21: GIS Analytical Functionality (VI) Readings on the topics Chapter 7, Aronoff (1993): pp Other readings Chapter 8, Bonham-Carter (1994): pp

Geog. 377: Introduction to GIS - Lecture 21 Overheads 2 7. Spatial Overlay: 7.1 What is spatial overlay? A process to combine two or more spatial data layers to produce a new data layer Most spatial queries require a spatial overlay operation Consists of two components: Overlaying location data - find homogeneous units over each of which the attribute values across the involved data layers are homogeneous (Homogeneous Units Figure)Homogeneous Units Figure Combining attribute data - Combine the attribute values over each of these homogeneous areas to produce a new value - the attribute values can be combined using simple logical operator or statistical methods or modeling Outlines

Geog. 377: Introduction to GIS - Lecture 21 Overheads Spatial overlay under raster (Map Algebra) Overlay location data: Pixels at same location in raster data layers with same pixel size and in a same coordinate system will line up very well. The overlay under the raster representation is simply done by combing the attribute values for the pixels at same location. (Figure 5.7 in Burrough)Figure 5.7 in Burrough Combine attribute data: (1) Arithmetic combination (for ratio and interval data) (Figure 7.15 in Aronoff)Figure 7.15 in Aronoff (2) Logical combination (all data types) The logical AND: Select area where soil is sandy AND vegetation is Pine The logica OR Select area where soil is sandy OR vegetation is Pine

Geog. 377: Introduction to GIS - Lecture 21 Overheads Spatial overlay under raster (Map Algebra) (continued …) The map algebra concept Output = (input1 OP input2) where OP is an operator (such as addition, multiplication) 7.3 Spatial overlay under vector (Polygon Overlay) (Figure 5.6 in Burrough)Figure 5.6 in Burrough Overlay location data Polygon intersection to form homogeneous units Major difficult issue: sliver polygons (splinters, spurious polygons) (Sliver Polygon Figure)Sliver Polygon Figure How to deal with sliver polygons: (a) determine if they are erroneous polygons - size of area - local knowledge (b) Removing the erroneous polygons - Merging based on shared length (Sliver Merging Figure)Sliver Merging Figure

Geog. 377: Introduction to GIS - Lecture 21 Overheads Spatial overlay under vector (polygon overlay) (continued) Combine attribute data 1) Attributes from the input data layers are carried over to the output data layer (Carried Over Attribute Figure)Carried Over Attribute Figure 2) Computing new attribute value through arithmetic or logical operations

Geog. 377: Introduction to GIS - Lecture 21 Overheads 6 Questions: 1. What is a spatial overlay? What are the typical steps involved in spatial overlay? 2. How is spatial overlay operation done under the raster representation? Why do we call overlay operation under raster “map algebra”? 3. How is spatial overlay operation done under the vector representation? Why is spatial overly under vector is more complicated than that under raster? 4. What is a sliver polygon? Are all sliver polygon erroneous? How would one determine which sliver polygon is erroneous or not? How would one remove an erroneous polygon? 5. Why are all attributes carried over into the output data layer under polygon overlay?