Adding some techniques and quality checks can help improve both the quality and the usability of your data…
Though its name sounds like an assurance of quality, the fact that data meets Quality Level B standards is no real guarantee that it’s suitable for your client’s job. Adding some techniques and quality checks can help improve both the quality and the usability of the data – thereby preventing the problems that set timetables back and push costs up.
Quality Level B is a standard for Subsurface Utility Investigations (SUI) adopted by the American Society of Civil Engineers. The US Department of Transportation Federal Highway Administration defines Quality Level B as involving the use of surface geophysical techniques to determine the existence and horizontal position of underground utilities.
Within the industry, using these techniques to find underground utilities is called “designating,” and the end product is two-dimensional mapping that is generally sufficient to support preliminary engineering goals. For example, Quality Level B data is a suitable basis for decisions on placement of storm drainage systems, footers, bollard posts for security and other design features to avoid conflicts with existing utilities. Engineers typically use this data to make slight adjustments in their design, which is far more cost effective than relocating utilities. But not all Level B data meets the same standards. First there are differences in quality depending on the techniques used to gather data. Second, there are differences in the ease with which you can reproduce and use the data for your project.
We’ve all probably seen these data quality issues first-hand – especially when project limits are expanded or there’s some other need to check work performed by an earlier designating team. We’ve all probably found and dealt with horizontal discrepancies, utility line omissions and mislocated utilities. All these problems in one preliminary phase of a project create even bigger problems for the overall project because they can delay schedules and increase costs. For example, when a data problem is discovered, the designating team has to verify all the previous work before they can move ahead. If the original data was gathered carelessly, then these additional checks often find not only discrepancies in the representation of marked utilities, but also additional buried utility lines. Previously marked lines need to be re-marked to separate them from the newly found lines. Poor quality data may lead to problems such as design revisions, change orders and construction delays.
To get first-rate results, thereby avoiding delays and cost overruns, make sure your Level B data has been gathered with all the following procedures in place. From the simple steps, like a project planning meeting, to high tech data collection that integrates real-time kinematic (RTK) GPS linked to a pen-based computer loaded with base mapping, these steps can help assure that you get the quality you expect.
Start with a kick-off meeting where everyone involved reviews the project limits, scope, time constraints, and safety issues. Follow that up by developing project tracking tools to set milestones and assign resources. Review all collected utility company research and highlight information that will be recorded. Especially in congested areas, transfer all utility company records to one overall drawing. That way the designator is carrying just the one combined drawing instead of a stack of records. This combined drawing will show the approximate location of all utilities in a certain area or at a congested intersection. As the final preliminary step, the project manager should take a walk through the site, looking for utilities that require special research or investigation and areas where GPS signals may be impeded by heavy foliage.
Investigate outside the project limits to identify and or clear all utilities that may enter your site. Color code like utility lines in areas where they crisscross or closely parallel each other to distinguish each line during collecting. This will be especially helpful if one crew marks the lines and another collects the marks. Trace signals one structure outside the project limits to verify that the target conductor has been accurately designated.
Field sketches (that will later be used as a guide for the survey crew and internal final review) should include the structure identification and the size and type of all lines. Make sure identification codes are consistent and clear. This information should be drawn on base mapping derived from aerial photography, tax maps or some other applicable mapping. To help the internal review process move smoothly, include the utility company’s reference numbers on your mapping as well as your own identification codes.
First, the GPS crew needs to set control. If the project manager found areas where GPS signals would be obscured, then the GPS operator can set additional control near these problem areas for use by a total station crew later.
Once control is set, the field crews start each day by occupying project control before they start data collection. Then the GPS operator uses the pen-based GPS-integrated CAD system to log coordinates, annotate line sizes and types while mapping the route of the target utility. As the surveyor walks along a designated utility line, the mapping on the hand-held computer screen moves, too. The cross hair on the screen indicates the operator’s location and serves as a quality check. Using this unique approach, the operator can see immediately whether the location of a painted mark is accurate in relation to the mapping. Other advantages include high accuracy and the ability to reproduce utility line locations without the use of an electromagnetic locator. Combining the ability to view the collected points in real time and to connect the line work in the field provides a field-to-finish product. This method marries electromagnetic technologies with the latest in data collecting and surveying equipment.
Codes and Field Notes
Start with a field code list that is suited for the client or project and develop a data collection system with the flexibility to annotate many different types of features. Use consistent field drafting procedures to make sure all mains and services have been marked or cleared. Note on the records with abbreviations, the structures that have been found, (F), and those that have not been found (NF). Be descriptive when detailing miscellaneous and non-typical structures. Use abbreviations such as SFB for See Field Book when a lengthy description of a unique situation is necessary.
Finally, compare all utility records, as-builts, highway and site plans to the final CAD plots and field mark-ups. Line sizes and types such as Fiber Optic or Duct should be noted. Casing sizes and length should be indicated or noted especially if they fall within a known area of conflict. Once the data has been checked internally, the supervisor should return to the field to check any unresolved areas or locations where field results differ greatly from records and to verify the accuracy of the collected data.
Benefits of GPS For Locating Utilities
- Reproduce the location of a line or structure and walk right to it by putting the unit in stakeout mode.
- The method is accurate (RTK accuracy is 3cm+/-) and cost-effective
- No line of sight is required
- GPS is not weather dependant/critical when tight deadlines are a factor
- GPS is fast. Site conditions dictate collection speed, but typically points can be collected at a two to three times greater number than conventional survey methods
- Can verify that sufficient data has been collected