ThemeRiver visualization depicts thematic variations over time within a large collection of documents. The thematic changes are shown in the context of a time line and corresponding external events. The focus on temporal thematic change within a context framework allows a user to discern patterns that suggest relationships or trends. For example, the sudden change of thematic strength following an external event may indicate a causal relationship. Such patterns are not readily accessible in other visualizations of the data. We use a river metaphor to convey several key notions. The document collection's time line, selected thematic content, and thematic strength are indicated by the river's directed flow, composition, and changing width, respectively. The directed flow from left to right is interpreted as movement through time and the horizontal distance between two points on the river defines a time interval. At any point in time, the vertical distance, or width, of the river indicates the collective strength of the selected themes. Colored ªcurrentsº flowing within the river represent individual themes. A current's vertical width narrows or broadens to indicate decreases or increases in the strength of the individual theme.
The Hanford Site is home to 177 large, underground nuclear waste storage tanks. Numerous safety and environmental concerns surround these tanks and their contents. One such concern is the propensity for the waste in these tanks to generate, retain, and periodically release flammable gases. This report documents some of the activities of the Flammable Gas Project Data Evaluation Task conducted for Westinghouse Hanford Company during fiscal year 1996. Described in this report are: 1) the results of examining the in-tank temperature measurements for insights into gas release behavior; 2) the preliminary results of examining the tank waste level measurements for insights into gas release behavior; and 3) an explanation for the observed hysteresis in the level/pressure measurements, a phenomenon observed earlier this year when highfrequency tank waste level measurements came on-line. This report, as is always the case, has benefited greatly from the comments of the peer, program and editorial reviewers: Tom Ferryman, Joe Brothers, Frank Ryan and Andrea Currie, respectively. Much of the fundamental research behind our understanding of the tank level and temperature measurements, as well as the machinery to handle these data, was supported by the Tank Surveillance group at Westinghouse Hanford Company, which at various times included Richard
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