GIS

Irreconcilable Spatial Differences

15 Aug, 2006 By: Kenneth Wong

Boundless combines GIS, GPS and CAD to align isolated survey plats


Robert Jones, a registered land surveyor, and Aaron Ford, a GIS manager, had been on the road so much lately they were having a hard time reconciling all the different time zones. They had just returned to their office in Brookfield, Wisconsin, after presenting at the ESRI International User Conference in sunny San Diego, California, and hadn’t eased back into their normal rhythm. When Cadalyst contacted them for an interview, they agreed to a teleconference at 10 AM Pacific time, but they were preparing to call at 8 AM Central time -- a full four hours ahead of schedule. “When you travel this much, your brain flip-flops for a few days,” Ford admits.

On the other hand, this team has found a way to reconcile spatially isolated survey data -- data that’s collected in different projects and not necessarily referenced to the same coordinate systems and physical monuments. They’ve done it by combining GIS, the backbone of spatial databases, with CAD and GPS, the surveyors’ preferred tools. Ford points out that the pairing of a surveyor and a GIS manager for the presentation is itself a clue. It signals an alliance between two seemingly isolated disciplines. That unlikely synergy is at the heart of their system, called Boundless.

Each dot represents a survey in Boundless, a high-accuracy, survey-grade GIS program from National Survey and Engineering (picture courtesy of National Survey and Engineering).

It’s All Relative

Boundless grew out of the daily work of National Survey and Engineering, a division of R. A. Smith and Associates, where Jones and Ford are both employed. Jones is the developer and sole inventor of the patent-pending application.

“Let’s get past the gobbledygook,” proposes Jones. “Simply put, if you know where one thing is in relation to a larger framework and you know where something else is in relation to the same framework, then you know where those two objects are in relation to each other.” So what’s preventing us from using this method to consolidate different survey records?

“Surveyors in engineering projects use CAD [to record boundary lines],” explains Jones. “These CAD files are associated with point files: X, Y and Z, followed by descriptions; or northing, easting and elevation, along with descriptions. They’re points on the surface of the earth, in a construction project or in an infrastructure. But those CAD files and point files generally exist in spatial isolation from one another. In other words, you have one survey plat here, another down the road, yet there’s no way to spatially relate the two to each other.” Even if both surveyors are drafting in AutoCAD and the two plats are next to each other, there’s no guarantee that they’ll perfectly align.

That, Jones, points out, is because survey projects are usually completed independently from one another, with different aims in mind. “The bearings and the coordinate systems used in those surveys may be different,” Jones says. “Then there’s also the legal description to consider. You plot one property in CAD according to its legal description, but the property next door also has to be drafted to correspond to its legal description. If those legal descriptions don’t match, there’s no way to spatially correlate the two survey plats.” Monumental Monumentation

The bane of the surveying profession, Jones explains, is the disappearance of site controls. “If you go out and survey a property,” he says, “you have to find something physical first. You’re describing a section of the surface of the earth; you need a reference point to begin with. It can be a pipe or a cross set down by a surveyor, a curve representing a road or a body of water -- something that serves as the point of beginning. That’s physical monumentation. But, over time, these monuments get destroyed or removed.”

So instead of relying exclusively on physical monumentation, Boundless also memorializes surveys within the NSRS (National Spatial Reference System). “We deploy high-accuracy GPS, tied to the CORS (Continuously Operating Reference Stations) network,” Jones clarifies. “And that [network] constitutes the NSRS.” GPS stations in the CORS network operate 24 hours a day, Jones observes. In a virtual reference system, which is a highly accurate, real-time CORS network, a surveyor can turn on a GPS device and, in about 30 seconds, obtain a reference point’s RTK (real-time kinematic) values, accurate within a centimeter of the NSRS. This becomes the larger framework, the uniform coordinate system within which all survey points and boundary lines can be spatially referenced.

The GPS requirement aside, Boundless creates no additional imposition on surveyors. Jones points out that survey project managers and CAD drafters will see no disruption to their workflow, no changes in their customary office procedures. If a client wants, surveyors can still report their measurements in CAD files referenced to local state-plane coordinate systems tied to certain physical monuments.

Users can export lines and points from unified surveys back into CAD or upload to field equipment (picture courtesy of National Survey and Engineering).

Minimizing Differences

Jones and Ford know many geospatial software solutions designed for reconciling survey data exist and they encountered some of them at the conference they’d just attended. Ford observes, “When [survey data] was brought into a GIS environment in these software programs, in some of them, the data was off by as much as two feet. While that may be acceptable for producing pretty maps, that’s not at all survey-grade accuracy.”

“The world is not flat -- it’s round,” Jones insists. Why does he feel the need to refute the medieval concept abandoned long ago? Because it’s important to keep that in mind when reconciling isolated survey plats displayed on a 2D surface. “If you have five surveys within two to three square miles, and you want to unify them,” Jones explains, “Boundless creates on the fly a local map projection that minimizes to the point of insignificance the difference between ground distances and the distances on that projected grid.” In other words, the projected grid lies on a plane so close to the ground elevation at the actual site that the differences between the two is well within the tolerance level in surveying. This method allows Boundless to align multiple survey plats without distorting the geometry as measured on the ground.

Multiple survey plats unified in ArcMap using Boundless (picture courtesy of National Survey and Engineering).

In the Field and On the Web

What good is Boundless if it’s bound to a desk while surveyors most likely to derive benefit from it are roaming the field? It makes much more sense for it to be accessible remotely. Hence, a Web-based Enterprise Version of Boundless was born.

The Web-based version, according to Jones, makes use of ArcGIS Server from ESRI. Though it was initially developed for internal use, from its inception, National Survey and Engineering recognized the technology’s commercial potentials. Jones foresees the NSRS becoming a more integral part of surveying. “At some point, in some counties,” he predicts, “tying the data to the NSRS may become a requirement.” If that happens, the Web-based Enterprise Version of Boundless will be an attractive solution. For a reasonable licensing fee, surveyors can simply log on to Boundless from a standard Web browser and begin using its features, leaving IT and software maintenance costs to the provider.

Beyond simple data collection, Jones also expects statistical analyses and data verification to become part of Boundless. “We do hundreds of surveys every year,” he says. “We also have to redo a significant number of them at some point, for land transactions, for example.” In those cases, Boundless becomes a time saver, “because we’re not hung up on lost physical monuments,” he points out.

National Survey and Engineering is currently enlisting beta testers. The Web version is expected to debut in the second quarter of 2007, according to Jones and Ford. Jones believes that “with the continued advances in high-accuracy GPS devices and virtual reference systems, the amount of spatial data surveyors can collect and manage is just going to explode.”


About the Author: Kenneth Wong


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