Date post: | 26-Dec-2015 |
Category: |
Documents |
Upload: | ella-melton |
View: | 218 times |
Download: | 3 times |
Discovering Interesting Regions inDiscovering Interesting Regions inSpatial Data SetsSpatial Data Sets
Christoph F. Eick
Department of Computer Science, University of Houston
1. Motivation: Examples of Region Discovery
2. Region Discovery Framework
3. A Fitness For Hotspot Discovery
4. Other Fitness Functions
5. A Family of Clustering Algorithms for Region Discovery
6. Case Studies:• Hot spot Discovery• Regional Association Rule Mining
7. Related Work
8. Summary
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Other Contributors to the Work Presented TodayOther Contributors to the Work Presented TodayRegion Discovery Framework• Banafsheh Vaezian (Master student, Department of Computer Science)• Dan Jiang (Master student, Department of Computer Science)Clustering Algorithms for Region Discovery• Jing Wang (Master student, Department of Computer Science)• Wei Ding (PhD student, Department of Computer Science)• Ji Yeon Choo (Master student, Department of Computer Science)• Rachsuda Jiamthapthaksin (PhD student, Department of Computer Science)Regional Association Rule Mining • Wei Ding (PhD student, Department of Computer Science)• Xiaojing Yuan (Faculty Member, College of Technology, UH)Regional Co-location Mining and Spatial Data Mining in General• Spatial Database and Data Mining Group (Shashi Shekhar, UMN) Software Platform and Software Design • Abraham Bagherjeiran (PhD student, Department of Computer Science)Other• Ricardo Vilalta (Faculty Member, Department of Computer Science, UH)• Shahab Khan (Faculty Member, Department of Geosciences, UH)
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
1. Motivation: Examples of Region Discovery1. Motivation: Examples of Region Discovery
RD-Algorithm
Application 1: Hot-spot Discovery [EVDW06]Application 2: Find Interesting Regions with respect to a Continuous VariableApplication 3: Find “representative” regions (Sampling)Application 4: Regional Co-location MiningApplication 5: Regional Association Rule Mining [DEWY06]Application 6: Regional Association Rule Scoping [EDWYK06]
Wells in Texas:Green: safe well with respect to arsenicRed: unsafe well
=1.01
=1.04
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
2. Region Discovery Framework2. Region Discovery Framework• We assume we have spatial or spatio-temporal datasets
that have the following structure: (x,y,[z],[t];<non-spatial attributes>) e.g. (longitude, lattitude, class_variable) or (longitude,
lattitude, continous_variable)• Clustering occurs in the (x,y,[z],[t])-space; regions are
found in this space.• The non-spatial attributes are used by the fitness
function but neither in distance computations nor by the clustering algorithm itself.
• For the remainder of the talk, we view region discovery as a clustering task and assume that regions and clusters are the same
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Region Discovery Framework ContinuedRegion Discovery Framework Continued
The algorithms we currently investigate solve the following problem:Given:A dataset O with a schema RA distance function d defined on instances of RA fitness function q(X) that evaluates clustering X={c1,…,ck} as follows:
q(X)= cX reward(c)=cX interestingness(c)size(c) with >1
Objective:Find c1,…,ck O such that:1. cicj= if ij2. X={c1,…,ck} maximizes q(X)3. All cluster ciX are contiguous (each pair of objects belonging to ci has to
be delaunay-connected with respect to ci and to d)4. c1,…,ck O 5. c1,…,ck are usually ranked based on the reward each cluster receives, and
low reward clusters are frequently not reported
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Challenges for Region DiscoveryChallenges for Region Discovery
1. Recall and precision with respect to the discovered regions should be high
2. Definition of measures of interestingness and of corresponding parameterized reward-based fitness functions that capture “what domain experts find interesting in spatial datasets”
3. Detection of regions at different levels of granularities (from very local to almost global patterns)
4. Detection of regions of arbitrary shapes
5. Necessity to cope with very large datasets
6. Regions should be properly ranked by relevance (reward)
7. Design and implementation of clustering algorithms that are suitable to address challenges 1, 3, 4, 5 and 6.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
3. Fitness Function for Hot Spot Discovery3. Fitness Function for Hot Spot Discovery
Cluster 1 Cluster 2 Cluster 3 Cluster 4 Cluster 5
|c| 50 200 200 350 200
P(c, Unsafe) 20/50 = 40% 40/200 = 20% 10/200 = 5% 30/350 = 8.6% 100/200=50%
Reward
Class of Interest: Unsafe_Well
Prior Probability: 20%γ1 = 0.5, γ2 = 1.5;R+ = 1, R-= 1;β = 1.1, =1.
10% 30%
1.1507
1 1.1200*
2
1 1.1350*143.0 1.1200*7
20
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
4. Fitness Functions for Other Region 4. Fitness Functions for Other Region Discovery TasksDiscovery Tasks
4.1 Creating Contour Maps for Water Temperature (Temp)
1. Examples in the data set WT have the form: (x,y,temp); var(c,temp) denotes the variance of variable temp in region c
2. interestingness(c)=
IF var(c,temp)>var(WT,temp)
THEN 0
ELSE min(1, log20(var(WT,temp)/var(c,temp)))
with being a parameter (with default 1)
3. Basically, regions receive rewards if their variance is lower than the variance of the variable temparature for the whole data set, and regions whose variance is at least 20 times less receive the maximum reward of 1.
Fig. 1: Sea Surface Temperature on July 7 2002
Var=2.2Reward: 48,5
Rank: 3
A single region and its summary
Mean=11.2
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Global Co-location: and are co-located in the whole dataset
Task: Find Co-location patterns for the following data-set.
4.2 4.2 Regional Co-location MiningRegional Co-location Mining
RegionalCo-location
R1
R2
R3
R4
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
A Reward Function for Binary Co-locationA Reward Function for Binary Co-location
Task: Find regions in which the density of 2 or more classes is elevated. In general, multipliers C are computed for every region r, indicating how much the density of instances of class C is elevated in region r compared to C’s density in the whole space, and the interestness of a region with respect to two classes C1 and C2 is assessed proportional to the product C1C2
Example: Binary Co-Location Reward Framework;
C(r)=p(C,r)/prior(C)
C1,C2 = 1/((prior(C1)+prior(C2)) “maximum multiplier”
C1,C2(r) = IF C1(r)<1 or C2(r )<1 THEN 0
ELSE sqrt((C1(r)–1)*(C2(r)–1))/(C1,C2 –1)
interestingness(r)= maxC1,C2;C1C2 (C1,C2(c))
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
How to Apply the Suggested MethodologyHow to Apply the Suggested Methodology
1. With the assistance of domain experts determine structure of dataset to be used.
2. Acquire measure of interestingness for the problem of hand (this was purity, variance, probability elevation of two or more classes in the examples discussed before)
3. Convert measure of interestingness into a reward-based fitness function. The designed fitness function should assign a reward of 0 to “boring” regions. It is also a good idea to normalize rewards by limiting the maximum reward to 1.
4. After the region discovery algorithm has been run, rank and visualize the top k regions with respect to rewards obtained (interestingness(c)size(c)), and their properties which are usually task specific.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
5. A Family of Clustering Algorithms for Region Discovery5. A Family of Clustering Algorithms for Region Discovery
1. Supervised Partitioning Around Medoids (SPAM). 2. Single Representative Insertion/Deletion Steepest Decent
Hill Climbing with Randomized Restart (SRIDHCR). 3. Supervised Clustering using Evolutionary Computing
(SCEC)4. Agglomerative Hierarchical Supervised Clustering (SCAH)5. Hierarchical Grid-based Supervised Clustering (SCHG)6. Supervised Clustering using Multi-Resolution Grids
(SCMRG)7. Representative-based Clustering with Gabriel Graph Based
Post-processing (SCEC+PGPP / SRIDHCR+PGPP)8. Supervised Clustering using Density Estimation
Techniques (SCDE)
Remark: For a more details: SCEC, SPAM, and SRIDHCR[EZZ04, ZEZ06]; [SCAH] and SCHG [EVJW04], SCMRG [EDWYK06],…+PGPP[CJCCE06]
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
SCAH (Agglomerative Hierarchical) SCAH (Agglomerative Hierarchical)
Inputs:A dataset O={o1,...,on}A distance Matrix D = {d(oi,oj) | oi,oj O },Output:Clustering X={c1,…,ck}
Algorithm:1) Initialize: Create single object clusters: ci = {oi}, 1≤ i ≤ n; Compute merge candidates based on “nearest clusters”
2) DO FOREVER a) Find the pair (ci, cj) of merge candidates that improves q(X) the most
b) If no such pair exist terminate, returning X={c1,…,ck} c) Delete the two clusters ci and cj from X and add the cluster ci cj to X d) Update inter-cluster distances incrementally e) Update merge candidates based on inter-cluster distances
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
SCHG (Hierarchical Grid-based)SCHG (Hierarchical Grid-based)
Remark: Same as SCAH, but uses grid cells as initial clusters
Inputs:A dataset O={o1,...,on}A grid structure GOutput:Clustering X={c1,…,ck}
Algorithm:1) Initialize: Create clusters making each single non-empty grid cell a cluster Compute merge candidates (all pairs of neighboring grid cells)
2) DO FOREVER a) Find the pair (ci, cj) of merge candidates that improves q(X) the most
b) If no such pair exist terminate, returning X={c1,…,ck} c) Delete the two clusters ci and cj from X and add the cluster c’=ci cj to X d) Update merge candidates: cX (MC(c’,c) MC(c, ci) MC(c, cj ))
1 2 3
4 5
6 7
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Ideas SCMRG (Divisive, Multi-Resolution Grids)Ideas SCMRG (Divisive, Multi-Resolution Grids)
Cell Processing Strategy
1. If a cell receives a reward that is larger than the sum of its rewards
its ancestors: return that cell.
2. If a cell and its ancestor do not receive any reward: prune
3. Otherwise, process the children of the cell (drill down)
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Representative-based Clustering
Attribute2
Attribute1
1
2
3
4
Objective: Find a set of objects OR such that the clustering X
obtained by using the objects in OR as representatives minimizes q(X).
Properties: Cluster shapes are convex polygonsPopular Algorithms: K-means. K-medoids
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Proximity Graph-Based Post-Processing[CJCCE06]Proximity Graph-Based Post-Processing[CJCCE06]
0 100 200 300 400 500 600 7000
50
100
150
200
250
300
350
400
450After Post Processing..# of clusters =5
0 100 200 300 400 500 600 7000
50
100
150
200
250
300
350
400
450Before Post Processing..# of clusters =94
Before After
Idea: Clusters with arbitrary shapes are approximated using unions of small convex polygons (that have been obtained by running a representative-based clustering algorithm, such as k-medoids)
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Pseudo Code PGPPPseudo Code PGPP
1. Run a representative-based clustering algorithm to create a large number of clusters.
2. Read the representatives of the obtained clusters.3. Create a merge candidate relation using proximity graphs.4. WHILE there are merge-candidates (Ci ,Cj) left whose
merging enhances q(X) BEGIN
Merge the pair of merge-candidates (Ci,Cj), that enhances fitness function q the most, into a new cluster C’=CiCj
Update Merge-Candidates: C (Merge-Candidate(C’,C) Merge-Candidate(Ci,C)
Merge-Candidate(Cj,C)) END
5. RETURN the best clustering X found.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Comparison of PGPP with K-meansComparison of PGPP with K-means
(a) K-means (b) Post-processing with
q1(X)
(c) Post-processing with
q2(X)
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
6a. Applications to Hotspot Discovery6a. Applications to Hotspot Discovery
Volcano Earthquake
Dataset Name # of objects # of classes
1 B-Complex9 3,031 2
2 Volcano 1,533 2
3 Earthquake-1 3,161 3
4 Earthquake-10 31,614 3
5 Earthquake-100 316,148 3
6 Wyoming-Poverty 493,781 2
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Experimental Results Experimental Results
DatasetAlgorithms SCAH SCHG SCMRG SCAH SCHG SCMRG
Parameters β = 1.01, η = 6 β = 3, η = 1
B-Complex9
Purity 1 0.998 1 1 0.997 0.863
Quality 0.974 0.974 0.957 0.008 0.044 0.002
Clusters 17 15 132 17 9 22
Volcano
Purity 1 0.692 0.979 1 0.692 0.885
Quality 0.940 0.091 0.822 1E-5 7E-4 1E-4
Clusters 639 56 311 639 31 221
Earthquake-1
Purity 1 0.844 0.938 0.853 0.840 0.814
Quality 0.952 0.399 0.795 0.004 0.086 0.006
Clusters 479 33 380 161 10 93
Earthquake-10
Purity DNF 0.840 0.912 DNF 0.834 0.807
Quality DNF 0.398 0.658 DNF 0.077 0.006
Clusters DNF 37 506 DNF 12 153
Earthquake-100
Purity DNF 0.842 0.909 DNF 0.837 0.808
Quality DNF 0.389 0.560 DNF 0.083 0.006
Clusters DNF 38 780 DNF 9 191
Wyoming
Purity DNF 0.772 0.721 DNF 0.769 0.661
Quality DNF 0.027 0.227 DNF 0 0.001
Clusters DNF 489 89 DNF 391 78
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Experimental EvaluationExperimental Evaluation
• SCAH outperforms SCHG and SCMRG when the penalty for the number of clusters is very low (=1.01, =6). However, when SCAH runs out of pure clusters to merge, it has the tendency to terminate prematurely; therefore, it does quite poorly when the objective is obtain large clusters (=3, =1).
• SCHG outperforms SCMRG and SCAH for =3, =1.
• SCMRG obtains better clusters than SCAH for the Volcano dataset for =1.01, =6, which can be attributed to the fact that SCMRG uses grid cells with different sizes.
• Avg. wall clocktime for smaller datasets SCAH:SCMRG/SCHG: 13:1/52:1
• SCAH is not suitable to cope with dataset sizes of 10000 and more, mainly because of the large number of distance computations, large numbers of clusters, and merge steps needed.
• The quality of clustering of SCMRG is strongly dependent on initial cluster sizes and on the look ahead depth.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Problems with SCAHProblems with SCAH
No look ahead:
Non-contiguousclusters:
XXX OOO OOO XXXToo restrictive definition of merge candidates:
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
6.b: Regional Association Mining6.b: Regional Association Mining
IF the well’s water is used by humans
and the well’s nitrate level is above 28.5
and the well’s fluoride level is between 0.005 and 0.195
THEN the well has dangerous levels of arsenic (support=0.5%, confidence=87%).
Example of an Association Rule:
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Why Regional Knowledge Important in Spatial Data Mining?Why Regional Knowledge Important in Spatial Data Mining?
• A special challenge in spatial data mining is that information is usually not uniformly distributed in spatial datasets.
• It has been pointed out in the literature that “whole map statistics are seldom useful”, that “most relationships in spatial data sets are geographically regional, rather than global”, and that “there is no average place on the Earth’s surface” [Goodchild03, Openshaw99].
• Therefore, it is not surprising that domain experts are mostly interested in discovering hidden patterns at a regional scale rather than a global scale.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Regional Association Rule MiningRegional Association Rule Mining
• Most data mining techniques are ill-prepared for discovering regional knowledge. For example, in traditional association rule mining, regional patterns frequently fail to be discovered due to insufficient global confidence and/or support.
• This raises the questions on how to identify interesting regions algorithmically, and how to measure the scope of a regional association rule
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Regional Association Rule Mining and ScopingRegional Association Rule Mining and ScopingSteps Regional Association Rule Mining
1. Find regions2. Mine regional association rules [DEWY06]3. Find the scope of discovered regional association rules[SDM06]
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Association Rule Scope Discovery FrameworkAssociation Rule Scope Discovery FrameworkLet a be an association rule, r be a region, conf(a,r) denotes the confidence of a in region r, and sup(a,r) denotes the support of a in r.
Goal: Find all regions for which an associate rule a satisfies its minimum support and confidence threshold; regions in which a’s confidence and support are significantly higher than the min-support and min-conf thresholds receive higher rewards.
Association Rule Scope Discovery Methodology:For each rule a that was discovered for region r’, we run our region discovery algorithm that defines the interestingness of a region ri with respect to an association rule a as follows:
Remarks:• Typically 1=2=0.9; =2 (confidence increase is more important than support increase)• Obviously the region r’ from which rule a originated or some variation of it should be
“rediscovered” when determining the scope of a.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Region vs. ScopeRegion vs. Scope
• Scope of an association rule indicates how regional or global a local pattern is.
• The region, where an association rule is originated, is a subset of the scope where the association rule holds.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Fine Tuning Confidence and SupportFine Tuning Confidence and Support
• We can fine tune the measure of interestingness for association rule scoping by changing the minimum confidence and support thresholds.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
7. Related Work7. Related Work
• In contrast to most work in spatial data mining, our work centers on creating regional knowledge and not global knowledge.
• A lot of work in spatial data mining centers on partioning a spatial dataset into “transactions” so that apriori-style algorithms can be used. We claim that our work can contribute to “finding such transactions” [DEWY06].
• Our work related to hotspot discovery has similarity to work in supervised clustering/semi-supervised clustering in that it uses class labels in evaluating clusters. Moreover, the goals of the algorithms presented are similar to hotspot discovery algorithms, a task that does not receive a lot of attention in spatial data mining, but more attention by scientists in earth sciences and related disciplines.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
8. Summary8. Summary
1. A framework for region discovery that relies on additive, reward-based fitness functions and views region discovery as a clustering problem has been introduced.
2. Families of clustering algorithms and measures of interested are provided that form the core of the framework.
3. Evidence concerning the usefulness of the framework for regional association rule mining amd hotspot discovery has been presented.
4. The special challenges in designing clustering algorithms for region discovery have been identified.
5. The ultimate vision of this research is the development of region discovery engines that assist earth scientists in finding interesting regions in spatial datasets.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
The Ultimate Vision of the Presented ResearchThe Ultimate Vision of the Presented Research
Spatial Databases
Data Set
DomainExpert
Measure ofInterestingnessAcquisition Tool
Fitness Function
Family ofClustering Algorithms
VisualizationTools
Ranked Set of Interesting Regions and their Properties
Region Discovery
Display
DatabaseIntegration
Tool
Architecture Region Discovery Engine
Family ofMeasures of
interestingness
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Why should people use Why should people use Region Discovery EnginesRegion Discovery Engines (RDE)(RDE)??
RDE: finds sub-regions with special characteristics in large spatial datasets and presents findings in an understandable form. This is important for:
• Focused summarization• Find interesting subsets in spatial datasets for further studies• Identify regions with unexpected patterns; because they are unexpected they deviate
from global patterns; therefore, their regional characteristics are frequently important for domain experts
• Without powerful region discovery algorithms, finding regional patters tends to be haphazard, and only leads to discoveries if ad-hoc region boundaries have enough resemblance with the true decision boundary
• Exploratory data analysis for a mostly unknown dataset• Co-location statistics frequently blurred when arbitrary region definitions are used,
hiding the true relationship of two co-occurring phenomena that become invisible by taking averages over regions in which a strong relationship is watered down, by including objects that do not contribute to the relationship (example: High crime-rates along the major rivers in Texas)
• Data set reduction; focused sampling
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Additional TransparenciesAdditional Transparencies
Additional Transparencies On Region DiscoveryNot Used in Lecture
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Experimental Results Volcano for Experimental Results Volcano for =1.01, =1.01, =6=6
SCAH
SCHG
SCMRG
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Using Gabriel Graphs to Determine Neighboring Using Gabriel Graphs to Determine Neighboring ClustersClusters
Volcano K = 100
Gabriel Graphs:(Ci, Cj) having an edge implies that Ci and Cj are neighboring
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Datasets UsedDatasets Used
• Obtained from Geosciences Department in University of Houston.
• The Earthquake dataset contains all earthquake data worldwide done by the United States Geological Survey (USGS) National Earthquake Information Center (NEIC).
• The modified Earthquake dataset contains the longitude, latitude and a class variable that indicates the depth of the earthquake, 0(shallow), 1(medium) and 2(deep).
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Datasets UsedDatasets Used
• Wyoming datasets were created from U.S. Census 2000 data.
• The Wyoming Modified Poverty Status in 1999 is a modified version of the original dataset, Wyoming Poverty Status.
• The Wyoming Poverty Datasets were created using county statistics. For each county, random population coordinates were generated using the complete spatial randomness (CSR) functions in S-PLUS.
• Then, the background information was attached to each individual county based on the county’s distribution for the class of interest. Finally, all counties were merged into a single dataset that describes the whole state.
Ch. Eick: Discovering Interesting Regions in Spatial Datasets
Datasets UsedDatasets Used
• Obtained from Geosciences Department in University of Houston.
• The Volcano dataset contains basic
geographic and geologic information for volcanoes thought to be active in the last 10,000 years
• The original data include a unique volcano number, volcano name, location, latitude and longitude, summit elevation, volcano type, status and the time range of the last recorded eruption.
• The Subset of the volcano dataset used in this thesis contains longitude, latitude and a class variable that indicates if a volcano is non –violent (blue) or violent (red).