Striking a Balance

February 1, 2004 By: Ian Fitzgerald GPS World

Absolute and Relative Accuracy within a Utility GIS

Geographic information systems (GIS) technology provides a sophisticated management tool for users of geospatial information. Within the utility sector, GIS has found many applications, including creation of multileveled maps of facilities and assets. However, its use brings complexity along with the benefits, and the adoption of high-accuracy GPS positioning as a mainstream technology for collecting data on utility features multiplies this complexity.

Despite the common impression of GIS as simply a sophisticated computer-based map, the technology yields more than just mapping. In organizations such as the Truckee Donner Public Utility District in Truckee, California, we see GIS used for engineering modeling, utility design, and facility relocation. Couple a GIS with a GPS's ability to accurately and precisely collect and navigate to feature data, and you have an extremely powerful tool within the utility sector.

This article discusses a common challenge faced by utility GIS managers and staff when they attempt to juggle the need to map utility features with enough separation from one another to allow for good cartographic representation and utility modeling, while maintaining the absolute accuracy of XYZ coordinates for facility location and integration into other GPS-powered applications. In other words, how can we take advantage of absolute GPS positioning, when the distances between mapped features are too small to allow the respective symbology to be displayed appropriately?

Figure 1 and 2.

The Problem with Precision

Relatively affordable mapping-grade GPS systems can collect data with meter-level positioning accuracies and often down to just six or seven inches. These accuracies are incredible, considering that power poles average 14 inches in diameter; electric vaults are typically 2 by 4 feet; and water valves are 6 inches in diameter. Consequently, establishing absolute locations of facilities for mapping purposes is more than just theoretically possible, it's practically achievable.

A stumbling block occurs when we map facilities, assets, and features at the scales typical for utilities: 1 inch equals 100 feet, or sometimes 200 feet. Even more challenging cartographically are the plot scales of 1 inch equals 1,200 feet for electrical feeder, or water/gas pressure zone maps. This can lead to the inability to use absolute values in GIS map displays. If we use absolute coordinates to place symbols on a map plotted for these scales, we often encounter symbology overstrike or, in layman's terms, an unreadable cartographic mess. Mapping with GPS-collected accuracies is just not conducive to all business needs within a utility GIS.

Utilities need readable cartographic representations of their infrastructure at many scales, while meeting the technical requirements for network connectivity and object-orientated business rules that allow for the true modeling and representation of a utility infrastructure, which is critical for such analysis as water/gas valve isolation or electrical outage predictions. Due to these needs, utilities must often map facility features with relative accuracy. Typical mapping guidelines in a utility environment may mean that water valves are always mapped 10 feet from any intersection or that electric cables within the same trench are drawn 4 feet apart and parallel to one another. Another common practice symbolizes an actual 4-by-4 foot electrical switch facility mapped as 25-by-25 feet in order to detail the switching components inside (Figure 1).

Figure 3: Using relative positions and related absolute positions simultaneously. Courtesy of Xcel Energy South, 2000

The Need for Absolutes

As great as the need to map facility features relative to one another, there is also the need to maintain absolute accuracy data for each facility feature. For utilities, requirements for absolute accuracy arise from four main concerns:

1. A need to relocate facilities, particularly underground facilities, in the future.

2. A need to design accurately and allow for precise placement considerations.

3. A need for accurate distance measurements among facilities and features.

4. A need for more precise modeling with elevation-sensitive users such as water and wastewater utilities.

These needs are in addition to the most obvious need of ensuring that the accuracy of GPS data collection, along with the costs spent collecting the data, is not lost.

Balancing Needs

Given these strong requirements of utilities, we face a definite need to balance absolute and relative accuracy within the utility GIS environment. So, how does one accomplish this? We'll explore three methods that allow for high quality cartographic representation and the ability to maintain highly accurate coordinates for future use.

Relative Point Features with Attributed Absolute Data. GIS comprises a two-pronged database system, consisting of a spatial element and an attribute element. With the inherent ability to model a feature spatially on a map and further define it with complementing attribute data, a GIS can represent a feature with relative and absolute accuracy simultaneously.

Considering the needs to map with symbology set for various scales while maintaining network connectivity and business rules, this approach uses relative accurate point features to satisfy the spatial element of a GIS design. Placement of relative features is accurate in the sense that, for example:

1. a water valve is to the north of a tee intersection
2. a pad-mount transformer is placed to the west of a junction enclosure
3. or a power pole is in the northwest corner of a lot.

These accuracies allow a user to recognize which feature they are observing, and give enough detail to establish the location of a facility or its role and position within any utility network.