GIS Systems And As-Built Data
O ver the last few decades municipalities and local governments have invested millions of dollars in Geographic Information Systems, known as "GIS." These elaborate systems serve as spatially oriented...
Over the last few decades municipalities and local governments have invested millions of dollars in Geographic Information Systems, known as “GIS.” These elaborate systems serve as spatially oriented databases for municipal infrastructure and assets.
While GIS investments have enabled municipalities to develop robust databases, many municipalities have not been recording ongoing changes to their infrastructure within their GIS systems in a timely and efficient way.
Historically, the engineering details of municipal infrastructure were captured on as-built drawings, which existed in the form of marked up hard-copy printed design drawings that were hung in a vault. Now that GIS systems are in place, municipalities and companies involved in engineering projects must adopt new procedures in order to leverage their full investment in GIS data. In other words, the job isn’t done until the data is in the GIS.
The GIS is the hub around which capital infrastructure and private land development projects revolve. Engineering and environmental companies use data from a GIS to develop their project plans and concepts. Municipal work crews use the GIS data for servicing roads, watermains, etc. And municipal planners use GIS data as a cornerstone for their work.
Need for accurate as-built data
Having accurate as-built data of infrastructure once it is constructed is very important. The level of accuracy is a function of how GIS data is used at a later date. Weighty decisions depend on the data. Yet municipalities often found that even the old hard-copy as-built submissions had errors.
A means of ensuring that as-built data is correct and
that those responsible are accountable for its accuracy must be put in place. The easiest way is for the municipality to take sole control of the process. As-built data submitted by private sector engineering consulting firms should be guaranteed for its correctness. When a municipality hires a consultant, the deliverable at the end of the project should include the legal document (record drawing), construction documents (construction plans, quantities and staking data), and as-built documents showing just the new infrastructure. The latter is the data source for the post-construction GIS entry. Accountability has always been in place via the engineer’s seal for the design documents, and accountability should also be in place for the as-built documents.
Current processes unreliable
At the present time, the processes used for collecting asbuilt data on infrastructure are inefficient and unreliable.
The engineers complete their designs in CAD programs based on AutoCAD or Microstation. But then, at the end of the project, the engineer or contractor takes hard copies of those drawings and marks them up to represent what was actually built. Many municipalities still support these “old school” hard copy submissions.
GIS technicians at the municipality then go to work to reproduce the drawings in the GIS environment by manually drafting the new infrastructure (roads, sewers, cadastral etc.) in the GIS. This work is clearly a duplication of effort, as often the hard copy drawings were plotted directly from a CAD environment that may even have been spatially coordinated to the correct coordinate zone.
A better scenario is when municipalities mandate that consultants, whether working directly for the municipality on capital infrastructure projects or working with land developers, have to submit CAD drawings. The CAD drawings are the original design drawings that have been “adjusted” to reflect what was actually constructed in the field.
Unfortunately, consultants are often reluctant to submit digital design drawings to a municipality for two reasons. One is because of potential liability — anybody can change the contents of a digital drawing.
The second reason is that public clients often require engineering consultants to adhere to a CAD standard when preparing drawings. The requirement for “stylized” data is so that municipal technicians doing the GIS data entry can more easily identify the distinct features in the submitted drawings. However, for consulting engineers the requirement can be very cumbersome and time consuming because (i) there is such a large amount of detail that goes into an engineering drawing, and (ii) consultants often work for more than one municipality.
Municipalities themselves have not given enough attention and devoted enough resources to this work. The period during which GIS was becoming prevalent in the construction industry coincided with a time when municipalities were downsizing and restructuring. Unfortunately ongoing GIS maintenance was left behind. Simply put, bodies must be put to the task. Dedicated resources must exist for the collection and reduction of as-built data to a GIS. At present, because this work is often done by technicians in a municipality’s design department, it is left for times when design and construction activity are low. The result can be a time lag of up to two years before the as-built data is input into the GIS.
New technologies to help
New survey and model based design technology gives opportunities for municipalities to address this GIS data collection problem. Sub-surface data (sanitary, storm, watermain and other utility) features are those that are changed most often from the engineer’s design during construction. The historic records are poor and so the crews in the field will often encounter unknown conditions. For this reason many municipalities choose to actually survey sub-surface features as they are constructed. The city of Las Vegas, for example, one of the fastest growing municipalities in North America, surveys pipes as they are built, and wires the as-built data back to the GIS technicians at the end of the day. Surface data such as road geometry are more often constructed as designed, and therefore can be sourced from design documents for the GIS data entry.
Today’s GPS survey equipment makes it easy to collect as-built data and should be applied to sub-surface utilities. Design drawings, if they are created to the required coordinate zone, could be sourced for surface features such as road geometry and cadastral data.
While the end result should be an as-built document that is representative of existing conditions once construction is completed, the accuracy of the data is a function of its future purpose — which most often will be planning and infrastructure maintenance. It is unlikely that an engineer will use 10-year–old data from a GIS upon which to base a detailed infrastructure design for construction. A rule of thumb for accuracy that many municipalities have adopted is 10 centimetres.
Finally, engineers should not be submitting marked up design drawings renamed “as-built” to municipalities once construction is complete. These engineering drawings contain annotations, title blocks, dimensions, existing conditions and the proposed works. The emphasis for post-construction submissions should be based on what is new, and just that.
If municipalities were to request as-built drawings that only include the new elements constructed, it would be much easier for consulting engineers to submit these drawings and adhere to a CAD standard. Also, the development of regional and even provincial standards for CAD and GIS submissions would be a real advantage.
The CAD submission should identify the infrastructure features and their spatial location. The second component of the submission is the attribute data such as invert elevations, pipe sizes and curb and gutter types. To submit this data in AutoCAD (Civil 3D and Land Desktop) could be accomplished very easily with block attributes or, better yet, Map 3D object data. A similar process could be applied to Microstation-based design systems.
Some of the more progressive municipalities are currently exploring LandXML (www.landxml.org) as a means to submit as-built da
ta because LandXML is not stylized data and therefore places no additional demand on consultants to adhere to a layer standard. Also LandXML interfaces with most GIS.
The requirement for as-built documents containing solely new infrastructure in a CAD environment is a very reasonable expectation, and one that municipalities should be willing to contract and pay for from their consultants. Someone has to do the work whether or not it is done internally or externally.
The bottom line is that any activity resulting in changes to infrastructure must be recorded in the GIS in an accurate and timely manner. Also, infrastructure data sources need to become less disparate. The Geographic Information System should be one-stop shopping for retrieving all municipal infrastructure data.
Andrew Walther, P. Eng. is a principal with APW Engineering of Vancouver. He helps organizations optimize their use of civil engineering and GIS technology. firstname.lastname@example.org