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Data Evolution
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Validating your GIS with Network Analysis
The diagram begins with accessing the facility information from GIS. Network analysis is then performed on the data set to identify potential problems. The results are then analyzed, and the problems are identified. The identified problems are grouped into two categories, data and design errors. Data errors are problems in the source information that are either inaccurate field information or missing information required for hydraulic analysis. Design errors are problems identified in the field where the GIS facility information is correct, but where network analysis has pinpointed potential weaknesses or breakdowns in the network’s design. Sorting the problems into these two categories helps identifi the action(s) needed to resolve each issue. If the problem is identified as a data error, the GIS source information is corrected and is once again run through network analysis until the remaining errors are within acceptable tolerances. If the problem is identified as a design error, then the modeler can
further evaluate the issue and determine if corrective actions are needed. To further understand how using network analysis tools within QA/QC procedures can validate and correct GIS data, it is important to understand the three components that make up hydraulic models. Each are discussed in more detail below.
Connectivity
The first of the three major components in evaluating the ability of GIS data to support hydraulic modeling is connectivity. Connectivity plays a critical role in network analysis by enabling the model to identifi which pipes tie into one another to determine the flow direction and pressure fluctuation. Many tools have been developed within GIS applications to ensure that connectivity is being maintained during data capture and maintenance updates. These tools are generally based on tracing routines which start at a specific point along a pipe or a node and trace outward until it reaches a point were the pipe dead ends. These tracing routines are very helpfid in identifying orphan pipes segments. Orphan pipes are individual pipe segments that are isolated from the hydraulic network. However, in a network model, identifying individual isolated pipe segments is just the first level of connectivity needed for accurate analysis of a hydraulic model. Additional connectivity requirements are needed. For example:
- Bottlenecks: Sections of pipe where reduction occurs from a large pipe diameter to a smaller pipe diameter. This results in a “bottleneck” effect where gas is so constrained at that point that pressure is unable to be maintained downstream due to the constriction.
- Subsystems: Small network of pipes disconnected from the main network. These subsystems can impact your analysis by not accurately representing all of the associated connections to the main model.
- Pressure system interconnections: This occurs when a low pressure system is tied back to the high pressure system without any device regulating or isolating the two separate pressure systems. This affects the model by not accurately representing the pressure and flow directions.
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