B.Tech. Project Presentation

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B.Tech. Project
Presentation

Intercepting a Moving Target in

Road Networks

by

Prateek Khatri

Under the guidance of

Prof. N. L. Sarda

The Problem


Given a road
-
network,
n

number of pursuers and one
evader, devise a strategy to coordinate all pursuers to
capture the evader.


Assumptions:


Speed of pursuer and evader not bounded


Pursuer receiving regular updates about evader position


Pursuer knows the initial position of the
evader

Introduction


Devising the strategy:
Here the aim is to develop a strategy
for pursuers considering the constraints of a road network.
Presently a simple shortest path strategy is implemented.
Developing an intelligent strategy for evader can help check
the efficiency of the pursuer strategy.


Simulation:
Here the aim is to develop a web based
interface for simulation and analysis of the strategies.
The interface will allow the user to select the simulation
parameters like starting nodes for pursuer and evader,
no. of pursuers and no. of evaders, etc.


Previous Works


The work by Parsons and Motwani have focussed on the
visibility based pursuit
-
evasion
in
graphs


Some others have advocated the use of randomized solutions
as the probability of pursuer catching evader increases


Most of the works have focussed on polygonal environments


None of the work encountered have focussed on road
networks specifically


Randomized strategy as given in [2] using RRTs focusses on
polygonal regions but can be adapted for graphs as well

Limitations of earlier works


Road networks are very different from the robotic
environments.


Dynamic constraints on fuel, roads,traffic conditions, number
of vehicles available


Implicit assumptions:


Bounded and polygonal environment


No constraints on paths


No constraints on number of pursuers

Strategies


Possible pursuer strategies:


Shortest path to evader at every update

(Implemented)


Dividing the area into n parts for n pursuers


Randomized Strategy


Heuristic based strategies: roadblocks, toll booths, etc.


Possible
evader strategies:


Random
(Implemented
)


Moving away from the initial point


Heuristic based strategies: crowded roads, narrow roads, hiding place, etc


Capture
Conditions:


Pursuer within some small distance of
evader (Implemented)


Pursuer
can see
evader (in case of line of sight)

Simulation


Discrete
-
event simulation has been implemented to test and
analyse the strategies.


The
problem is simulated with one pursuer and one evader with
following strategies:


Pursuer


Shortest path at every update


Evader


Random run and moving
far away from the
initial position


Capture condition


evader within some distance of pursuer


Assumptions


Pursuer needs random updates to follow evader


Total number of events in the simulation can not be more than 1000


Simulation is over if it one of the two conditions are satisfied:


Evader is caught


Total number of events become more than 1000





Implementing Simulation

Implementing Simulation

Visualization


A web
-
based visualization software is
developed to monitor and analyze the process


User can set the simulation parameters, can
select the initial nodes for pursuer and evader.


Developed using JSP, Servlets and OpenLayers

DEMO

Visualization workflow

index.jsp


user selects the map


sets the no. of pursuers and
evaders


map id is passed to the
controller class

controller.java


fetches the map data using the
map id.


prepares a mapInfo obj


stores the obj in the session

map.jsp


user selects the initial nodes


sets the simulation parameters


run the simulation

controller.java


prepares a Simulate obj.


sets the simulation params


run the simulation


stores the simObj in the session

showSimResult.jsp


user monitors the simulation
process


user can control the process by
advancing the simulation

Manual Tasks

Extracting
Nodes


Nodes (joints in a MULTILINESTRING where LINESTRINGS meet), needs to be extracted using
some external software, e.g.:
QuantumGIS

or a specific program written for this

SHP to GML
conversion


Convert the map
shapefile

to map<id>.xml


Convert the map
-
nodes
shapefile

to map<id>nodes.xml

PostGIS
tables


Create a table map<id> with following columns:


gid



road ids in the map


lines


geometry column containing roads as MULTILINESTRING geometry


Create a table map<id>_nodes with following columns:


gid



original road ids in ascending order


nodes


geometry column containing nodes as POINT geometry

Display

Layers
(vector)

Map Layer

Map
Nodes
Layer

Pursuer
start point
layer

Evader
start point
layer

Pursuers
end point
layer

Evader
end point
layer

Pursuer
path layer

Evader
path layer

Layer displays
the map network

Layer displays
nodes in the map

Shows the
starting
positions

Shows the
path

Shows the
current
positions

Results: capture time vs. number of
pursuers


Map used: Hyderabad Road Network


Difficulty of taking into account all the factors
responsible in chase is avoided by measuring
the simulation time over 10 and 20 simulation
runs and averaging the results



Result: measured over 10 simulation
runs

Results: measured over 20 simulation
runs

Future Work


Developing heuristic based strategies for both pursuer
and evader


Incorporating the road constraints


Automating all the tasks in required in the preprocessing
for visualization


Use of Raster layers instead of vector layers for displaying
map will speed up the process

References


[1] Theory and Applications of Graphs, chapter Pursuit
-
evasion in a
graph. Springer Berlin / Heidelberg, 1978.


[2] A. AlDahak and A. Elnagar. A practical pursuit
-
evasion algorithm:
Detection and tracking. In Robotics and Automation, 2007 IEEE
International Conference on, pages 343
-

348, April 2007.


[3] W. Herbert and F. Mili. Route guidance: State of the art vs. state of
the practice. In Intelligent Vehicles Symposium, 2008 IEEE, pages
11671174, June 2008.


[4] L. J. Guibas, J.
-
C. Latombe, S. M. LaValle, D. Lin, and R. Motwani.
A visibility
-
based pursuit
-
evasion problem. In Intl. J. of
Computational Geometry Applications, volume 9, pages 471
-

493,
1999.