Chapter 8 Geocomputation Part A: Cellular Automata (CA) & Agent-based modelling (ABM) www.spatialanalysisonline.com

Geocomputation “the art and science of solving complex spatial problems with computers” www.geocomputation.org Key new areas of geocomputation: Presentation 8A: Geosimulation (CA and ABM) Presentation 8B: Artificial Neural Networks (ANNs); & Evolutionary computing (EC) 3rd edition www.spatialanalysisonline.com

Geocomputation Many other, well-established areas: Automated zoning/re-districting (e.g. AZP) Cluster hunting (e.g. GAM/K) Interactive data mining tools (e.g. brushing and linking, cross-tabbed attribute mapping) Visualisation tools (e.g. 3D and 4D visualisation, immersive systems… some also very new!) Advanced raster processing (e.g. ACS/distance transforms, visibility analysis, image processing etc.) Heuristic and metaheuristic spatial optimisation, …. and more! 3rd edition www.spatialanalysisonline.com

Geocomputation: Geosimulation For the purposes of this discussion: Geosimulation includes Cellular automata (CA) Agent-based modelling (ABM) Geosimulation is particularly concerned with Researching processes Identifying and understanding emergent behaviours and outcomes Spatio-temporal modelling 3rd edition www.spatialanalysisonline.com

Geocomputation: ANNs In the next presentation on geocomputation: ANNs discussed include Multi-level perceptrons (MLPs) Radial basis function neural networks (RBFNNs) Self organising feature maps (SOFMs) ANNs are particularly concerned with Function approximation and interpolation Image analysis and classification Spatial interaction modelling 3rd edition www.spatialanalysisonline.com

Geocomputation: Evolutionary computing In the next presentation on geocomputation: EC elements discussed include Genetic algorithms (GAs) Genetic programming (GP) EC is particularly concerned with Complex problem solving using GAs Model design using GP methods 3rd edition www.spatialanalysisonline.com

Cellular automata (CA) CA are computer based simulations that use a static cell framework or lattice as the environment (model of space) Each cells has a well-defined state at every specific discrete point in time Cell states may change over time according to state transition rules Transition rules that are applied to cells depend upon their neighbourhoods (i.e. the states of adjacent cells typically) 3rd edition www.spatialanalysisonline.com

Cellular automata State variables Spatial framework typically binary (e.g. alive/dead), but can be more complex may have fixed (captured) states Spatial framework typically a regular lattice, but could be irregular boundary issues and edge wrapping options Neighbourhood structure Typically Moore (8-way) or von Neumann (4-way) Typically lag=1 but lag=2 .. and alternatives are possible Transition rules Typically deterministic but may be more complex Time treated as discrete steps and all operations are synchronous (parallel not sequential changes) 3rd edition www.spatialanalysisonline.com

Cellular automata Neighbourhood structure Typically Moore (8-way) or von Neumann (4-way) Typically lag=1 but lag=2 .. and alternatives are possible 3rd edition www.spatialanalysisonline.com

Cellular automata Example 1 – Game of life State variables: cells contain a 1 or a 0 (alive or dead) Spatial framework: operates over a rectangular lattice (with square cells) Neighbourhood structure: 4 adjacent (rook’s move) cells State transition rules: time tntn+1 Survival: if state=1 and in neighbourhood 2 or 3 cells have state=1 then state  1 else state  0 Reproduction: if state=0 but state=3 or 4 in neighbouring cells then state  1 Death (loneliness or overcrowding): if state=1 but state<>2 or 3 in neighbourhood then state  0 3rd edition www.spatialanalysisonline.com

(still evolving – to density 4%) Cellular automata Life (ABM framework): Click image to run model (Internet access required) t0 35% cell occupancy Randomly assigned tn – evolved pattern (still evolving – to density 4%) 3rd edition www.spatialanalysisonline.com

Cellular automata Example 2 – Heatbugs State variables: Cells may be occupied by bugs or not Cells have an ambient temperature value 0 Bugs have an ideal heat (min and max rates settable) – i.e. a state of ‘happiness’ State transition rules: time tntn+1 Bugs can move, but only to an adjacent cell that does not have a bug on it Bugs move if they are ‘unhappy’ – too hot or too cold (if they can move to a better adjacent cell) Bugs emit heat (min and max rates settable) Heat diffuses slowly through the grid and some is lost to ‘evaporation’ 3rd edition www.spatialanalysisonline.com

Cellular automata Heatbugs (ABM framework): Click image to run model (Internet access required) 3rd edition www.spatialanalysisonline.com

Cellular automata Example geospatial modelling applications: But.. Bushfires Deforestation Earthquakes Rainforest dynamics Urban systems But.. Not very flexible Difficult to adequately model mobile entities (e.g. pedestrians, vehicles)… interest in ABM 3rd edition www.spatialanalysisonline.com

Agent-based modelling Dynamic systems of multiple interacting agents Agents are complex ‘individuals’ with various primary characteristics, e.g. Autonomy, Mobility, Reactive or pro-active behaviour, Vision, Communications capabilities, Learning capabilities Operate within a model or simulation environment Time treated synchronously or asynchronously CA can be modelling using ABM, but reverse may be difficult Bottom-up rather than top-down modelling 3rd edition www.spatialanalysisonline.com

Agent-based modelling Sample applications: Archaeological reconstruction Biological models of infectious diseases Modelling economic processes Modelling political processes Traffic simulations Analysis of social networks Pedestrian modelling (crowds behaviour, evacuation modelling etc.) … 3rd edition www.spatialanalysisonline.com

Agent-based modelling Example 1: Schelling segregation model Actually a CA model implemented here in an ABM framework. Agents represent people; agent interactions model a social process Spatial framework: Cell based State variables: grey – cell unoccupied; red – occupied by red group; black – occupied by black group Neighbourhood structure (Moore) State transition rules: If proportion of neighbours of the same colour x% then stay where you are, else If proportion of neighbours of the same colour <x% then move to an unoccupied cell or leave entirely 3rd edition www.spatialanalysisonline.com

Agent-based modelling Schelling (ABM framework): Click image to run model (Internet access required) 3rd edition www.spatialanalysisonline.com

Agent-based modelling Example 2: Pedestrian movement Realistic spatial framework Multiple passengers arriving and departing Multiple targets – ticket machines, ticket booths, subway platforms, mainline platforms, shop, exits … Free movement with obstacle avoidance 3rd edition www.spatialanalysisonline.com

Agent-based modelling Pedestrian movement: Click image to run model (Internet access required) 3rd edition www.spatialanalysisonline.com

Agent-based modelling Advantages of ABM Captures emergent phenomena Interactions can be complicated, non-linear, discontinuous or discrete Populations can be heterogeneous, have differential learning patterns, different levels of rationality etc Provides a natural environment for study Spatial framework can be complex and realistic Flexible Can handle multiple scales, distance-related components, directional components, agent complexity etc 3rd edition www.spatialanalysisonline.com

Agent-based modelling Disadvantages of/issues for ABM What is the real ‘purpose’ of model? What is the appropriate scale for research? How are the results to be interpreted? How robust is the model? Can the model be replicated? Can the results be validated? Are behaviours/patterns observed likely to occur in the real world? How much is the outcome dependent on the model implementation (design, toolset, parameters etc.)? 3rd edition www.spatialanalysisonline.com

Agent-based modelling Choosing a simulation/modelling system Ease of development Size of user community Availability of support Availability of demonstration/template models Availability of ‘how-to’ materials and documentation Licensing policy (open source, shareware/freeware, proprietary) 3rd edition www.spatialanalysisonline.com

Agent-based modelling Choosing a simulation/modelling system Key features Number of agents that can be modelled Degree of agent-agent interaction supported Model environments (and scale) supported (network, raster, vector) Multi-level support (agent hierarchies) Spatial relationships support Event scheduling/sequencing facilities 3rd edition www.spatialanalysisonline.com

Agent-based modelling Major simulation/modelling systems open source: SWARM, MASON, Repast shareware/freeware: StarLogo, NetLogo, OBEUS) proprietary systems: AgentSheets, AnyLogic 3rd edition www.spatialanalysisonline.com