Keep Utility Lines Out of the Twilight Zone16 May, 2006 By: Kenneth Wong
PUD prevents data distortion with a combination of new policy and technology rules
Some water, gas and electric pumps seem to disappear into the Twilight Zone -- a mangled spatial dimension that doesn't exist anywhere. Those in the field swear something is there, but back at headquarters, supervisors can't seem to pinpoint locations. Sometimes it's the other way around: A team of technicians dispatched to service a line can't locate the pump station, even though it's clearly indicated in the database.
No, a chunk of land has not evaporated into thin air. The reason for the confusion is incorrectly registered data. When a system merges as-built CAD data, survey data in different state plain coordinate systems, original land information and aerial photographs, locations don't always line up perfectly. When adjustments are made, a nudge or two on the computer screen can translate into a significant distance on the ground.
The service domain of Truckee Donner PUD (Public Utility District) in Northern California spans an area of approximately 44 square miles, at elevations of 5,000-8,000ft. In size alone it's no Alaskan plains, but its winter can be just as harsh, as the ill-fated pioneers of the Donner Party discovered in 1846. You don't want a service technician trudging through ten feet of snow to locate an elusive pump station. To prevent its field staff from stumbling into the Twilight Zone, the PUD devised a workflow -- a combination of policy and technology -- to manage the integration of GIS and CAD data.
|Out in the field along Donner Pass Road, technicians can't waste time trying to pinpoint an elusive pump station.|
Reality Isn't Absolute
Ian Fitzgerald, GIS coordinator at Truckee Donner PUD, discourages thinking in absolute terms -- that is, the absolute coordinate system (where x,y,z is set to 0,0,0) in which most surveyors and developers tend to collect their data. "It doesn't mean anything," he says. "All information is relative to [the hypothetical project space], but it's not relative to anything in the real world beyond the project itself. It might be horizontal data, but there's no vertical data to go with it. It doesn't take into account the curvature of the earth. So when you move that data into a larger spatial dataset, a lot of that information gets warped." And that contradicts the fundamental tenets of GIS, because, Fitzgerald points out, "GIS is very spatial. Everything must have a vertical or horizontal coordinate system."
Fitzgerald believes the prevalent practice among surveyors may change if they become more aware of how the collected data is used beyond the project's lifecycle. "It's not just a survey record to go into the county's records," he points out. "You might want to know zoning classification, how a parcel develops over time or who owns the parcel. You might do slope analysis, not just on this particular parcel but on the entire area, mile by mile."
Stop the Arbitrary Name-Calling
A famous axiom of the computing world is, "Garbage in, garbage out." Truckee Donner PUD has implemented an "as-built policy" to control what goes into its geospatial database. The district requires consulting engineers, contractors and other project proponents to tender a deposit beforehand. The deposit is refunded after successful delivery of as-built drawings in hard copies and electronic copies. If the data submitted doesn't comply with the prescribed layer-naming conventions, georeferencing principles and other guidelines, then the district deducts from the refund the cost of labor required to correct the files.
By discouraging arbitrary formatting practices, the as-built policy minimizes the workload for the district's GIS staff. At the same time, Fitzgerald says, "We don't want to increase the workload of the developers and surveyors either," especially because such increases are reflected in the final invoice submitted to the PUD. Because most developers and surveyors work in CAD, the district developed a GIS-to-CAD export utility to provide consultants with the correct horizontal and vertical coordinates for the project.
No More Extraordinary Exuberance
"CAD layering can be extraordinarily exuberant," Fitzgerald remarks. To maintain consistency in layering schemes, the PUD supplies consultants with a layer-structure template. But this is more than an exercise in minimalism. "Now, when we get the data back, we know exactly what each layer is supposed to be," says Fitzgerald. A consistent layer structure is necessary to write a program for automatically extracting the geospatial information embedded the CAD files submitted. "That's hours of labor saved in cleaning up those files," Fitzgerald points out. "And the data is no longer warped or lost, so there are fewer errors."
The PUD has standardized its geospatial workflow in ESRI products, which provide tools for loading CAD data and overlaying them on geospatial data. The District's export utility, developed with ESRI's assistance, lets administrators define feature classes, define layering structures and export the information in CAD-compatible DXF format.
|The user interface of the export utility was developed by Truckee Donner Public Utility District with ESRI's assistance to exchange data with consultants using CAD.|
Demand Drives Interoperability
Don Kuehne, a technical product manager for CAD interoperability at ESRI, observes that this as-built policy might be difficult to implement for some utility firms, especially if they don't have enough leverage to demand compliance from potential consultants.
"A lot of utility firms say they can't do that because the contractors won't buy into it," says Kuehne. On the other hand, he remarks, "This approach can certainly be useful for organizations that have internal GIS and CAD departments. Adopting a workflow like this goes a long way."
Fitzgerald acknowledges that although Truckee Donner PUD's as-built policy is strict, but says he has seen much stricter policies in other districts.
Kuehne believes that the demand for GIS-CAD integration may force competing software vendors to work together, willingly or not. "Take a look at Bentley's ArcGIS Connector product," he says. "It was acquired from Haestad Methods and includes ESRI software, to create a GIS interface for AutoCAD. Now that's my poster child for interoperability."
"Integrating CAD into Enterprise GIS," a paper by Steve Murphy, a mapping technician at Truckee Donner PUD, can be downloaded from ESRI's online library. In addition, Kuehne maintains a blog called GIS CAD Interoperability. For more on this topic, also read "Spatial Technologies: GIS for Utilities" from the November 2004 issue of Cadalyst.