USGS Coastal & Marine Geology

GIS Analysis
Results and Conclusions
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GIS tools to analyze these data sets were selected to meet several requirements. The time continuity of the entire navigational trackline needs to be preserved from the start to the end of the survey to support queries based on navigation time. In this study specifically, time-delimited (equipment on-off) sections of the trackline should be extractable, even if bisected by looping or crossing tracklines. Mixed zero-length and non-zero-length (photograph) observations should also be extractable.

The Arc/Info point and line data models alone do not meet all these requirements. Navigation tracklines are best represented as a linear feature rather than as individual points. The start-to-end continuity of a navigation trackline is disrupted if the navigation is assembled as a line coverage. Building a line coverage sorts and groups the arcs based on their spatial relationship to each other and thus does not maintain the time-sequential nature of the vertices that make up the arcs.

Extracting particular intervals (arcs) of the trackline between selected time fixes is cumbersome with the line and point models. The inability to attach attribute information to individual vertices of the arc requires that a point feature be created to hold the time attribute for each of the navigation fixes. To extract particular time intervals of the trackline the points must be selected and then assembled as an arc. Zero-length arcs are not allowed, so the single-instant photographic observations need to be dealt with separately as points.

In contrast, the Arc/Info dynamic segmentation data model, which consists of route, section, and event features, does preserve the time sequence of the navigation and provides a means for extracting line or point data based on time. The entire navigation log (reconfigured in the GENERATE>ROUTE input format) is built as a single route with time as the measure; sections are defined between every two time consecutive fixes (sections are all one minute long) (Table 5, mkroute.aml). This section structure allows changes in hour or day from one record to the next--which calculate as great differences in measures--to be absorbed without disrupting the route (Table 4). An INFO point event table provides access to attributes of single locations (points) on the route. Each one-minute entry in the navigation log qualifies as a point event. The point event table of the navigation log uses time as the measure and the route number (1) as the event key (Table 5, mkpointeve.aml). The attributes from the point event table are related by the EVENTSOURCE command to times on the route for display and query. For example, hourly and 10-minute fixes can be selected from a MINUTE item redefined from the navigation time (Figure 4).

The subsegments of the entire navigation log defined by start and end times in Tables 2 and 3 can be extracted by converting the tables to INFO line-event tables that include start and end times plus the route number. Similar to the point events, line events are related by the EVENTSOURCE command to times on the route for display and query (Table 5, mklineeve.aml). Line events do not balk at zero length, so all the data from the camera observations are preserved. Once the extraction is completed the line events are converted to arc coverages because the event structure is no longer necessary and event display commands are very slow (Table 5, plotrt.aml, event2arc.aml, plottrk.aml). Oddly, but fortunately, the zero-length line events survived the conversion to arcs. To be able to plot zero-length observations, the arcs were converted back to 1-minute fixes (each arc vertex was one minute on the navigation route) by ARCPOINT; the point attribute data are accessed by a RELATE to the line event table (Table 5, morphpt.aml).


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Maintainer: Florence L. Wong
Last modified: 01 Oct 97.
