Mapping under Cover

By Bradley Bryan and Gavin Schrock
“The tree which moves some to tears of joy is in the eyes of others only a green thing which stands in their way” —William Blake
Those darn trees. Mind you, we love trees; they serve to please the eye, renew as a resource, clean our air, shade our lives, and spark joy and imagination. But it is likely that every mapper or surveyor has been guilty of cursing them from time to time, especially when they find that trees limit their use of the positioning tools that global navigation satellite systems (GNSS) enable.

GNSS capabilities have improved dramatically, and our use of this amenity has increased, as well. There has been a renaissance in use of GNSS for asset inventory and resource mapping, driven not only by the improved tools in GNSS and geospatial data management but also by necessity born of economic drivers. Funding woes have pressed both public and private entities, in such segments as transportation, utilities, and natural resources, to seek better management of existing assets as the replacement cycles for infrastructure are getting longer. The life of infrastructure can be extended greatly through careful monitoring, maintenance, rehabilitation, and management. Next, we’ll examine a particular asset management project that is overcoming canopy challenges.

Know Your Assets

To best manage assets, you first have to know what assets you have. In the early days of geographic information systems (GIS), assets and resources had to be rapidly inventoried, modeled, or “assumed” to provide wide enough dataset coverages (or themes) to enable any kind of enterprise-wide asset analyses and management. People used lower precision data-collection methods because of compressed timelines and budgets. Aerial photogrammetry, meter+ GPS, sampling and modeling: these technologies and methods are amazing in their own right and wholly sufficient for many types of infrastructure and data, but they greatly lack in precision for others.  The need for higher precisions for some features, like the need for reliable elevation data for gravity systems such as drainage, sewer, and water, is not lost on these systems’ managers.

It was only quite recently that the capabilities for rapid, precise, and widespread inventory of assets has been truly cost effective.  The advent of such tools as high-precision mapping GNSS units and corrections from real-time networks (RTN) herald this new asset-mapping renaissance. RTN are seeing a boom in asset mapping users; the Washington State Reference Network (WRSN) counts over half of all new users as asset-mapping related, with public and private utilities at the fore.

Assets, like the infrastructure of utilities, span wide areas to reach customers, facilities, and resources. Whether infrastructure is placed on the surface, underground, or overhead, it is often impractical to negotiate and pay for access across private property, so the standard is to place the infrastructure within the public rights of way along roads. A lot of features can be jammed in/over/under those narrow rights of way: road, signage, utilities, walkways, and trees. 

Meeting Canopy Challenges

Can GNSS be effective under canopy? What kind of results would a user expect under different types of canopy, and how do you classify canopy for type and density? How do you determine if you are getting decent results while working under canopy? What kind of research has been done in these matters? And what types of hardware and software innovations have improved the prospects for working under canopy? (For more analysis of the above questions, refer to the extended online companion to this article on PSM’s website.)

As an example of how capabilities to map under a canopy have progressed, Seattle Public Utilities has been tackling a multi-year asset inventory of drainage features and urban creeks that are almost exclusively in moderate- to high-canopy corridors of the city. The project has been trying out many new products along the way, and team members decided in late 2011 to try one that touted some advantages specifically in working under canopy. Some of the more challenging corridors were chosen for tests that included good vertical control established previously with digital levels. We told them to bring it on.

The Mapping Ninja

The South Fork of Thornton Creek in north Seattle is a salmon-bearing urban tributary to Lake Washington. A damp morning in October of 2011 was chosen for the tests, in a deep channel flanked on the south by a steep six-foot rock embankment, with a two-story house rising over that and one of the largest native maples in the city above.

The opening chords to ACDC’s “Thunderstruck” echo down the creek; this is the ringtone of Mel Philbrook, aka the “Mapping Ninja,” who answers the call to help yet another customer in another far-flung place with a mapping conundrum.  Mel is well known to Seattle Public Utilities and many around the country, having done stints with various dealers, public utilities, and local governments as a mapping and GIS specialist and as a trainer for Department of Defense clients. Mel is now serving as a field applications engineer—a kind of “parachute him in there to figure things out” guy for Trimble’s mapping division.

He was eager to take the challenge and have his latest tool put to the “Seattle canopy hell” test. The Geo 6000 is a fully functioning, dual-frequency unit that can function as a handheld, like the Topcon GRS and GMS series and Leica Zeno. And like the other handhelds, his unit can also be run on a pole with an external antenna for optimal performance (it helps to get that antenna up higher and out of the multipath and sky blockage caused by holding it at your waist).

We asked Mel to observe spots we had previously mapped with total stations or levels but had not been able to observe even with our surveying rovers. At the creek location mentioned above, he was able to maintain the vertical element down to a few centimeters. We were still skeptical, so we went on to half a dozen other canopy-hell spots, like a benchmark at the foot of a three-foot wall to the southwest—and he kept hitting our values from 1-3cm. How in the heck is this happening?

Of course, there are trade secrets that no manufacturer might share, but Mel was able to tell us that this particular gizmo uses Floodlight, which is a three-pronged approach. It is comprised of multi-constellation positioning, advanced tracking algorithms, and filters and altitude-constrained positioning.  

An essential piece to attitude-constrained positioning is an electronic barometer that is used to improve accuracy and limit position outages. Mel did teach us to turn off the unit in the truck between sites, as leaving it on might erroneously bias the barometer. The best method is to “acclimate” the barometer out in the open at each site, then slowly walk in under the canopy.

There is a “decimeter” version of the unit and now a “centimeter” version. We hope to implement this type of solution and the others we are testing in the ongoing asset-mapping project. We have avoided some areas because of heavy canopy, but hopefully not any more.

SWAMP: a Challenging Asset-Inventory Program

Envisioned four years ago by Scott Reese, a GIS specialist with Seattle Public Utilities, the Surface Water Asset Management Program (SWAMP) started as a pilot project to see if the “holes” in the city inventory of drainage assets could be filled in precisely and cost effectively.  The pilot project has now turned into a fully fledged, high-tech-leveraging program, fulfilling not only the original mandate but also providing much needed data for many other asset-management uses.Since its inception in 1972, the National Pollutant Discharge Elimination System (NPDES) permit program has been a driver for many urban-asset management programs, grants, and mapping. For Seattle Public Utilities, the NPDES requirements provided some, but not all, of the impetus to initiate SWAMP, and also the need to manage urban flooding and combined sewer outfalls.

The city had periods of rapid growth, some of these during periods when sensitivity to drainage concerns may not have been a hot topic. “The city annexed a lot of areas with primitive drainage structures like sandboxes,” says Reese, “and usually there were no records of the drainage features.” Lacking complete inventory in a hilly, rainy city proves a challenge to drainage management. Scott also explains how some areas of the city have been historically underserved regarding drainage, and the program is also hailed as fostering equity for city customers.

Scott’s approach is to have his crews systematically walk the city street corridors, quarter section by quarter section, following what records exist, updating feature data, adding better horizontal and vertical positions, and adding new features as they are located. Scott’s SWAMP team gives high priority to higher precision. Working directly with the licensed surveyors at the city, field procedures were developed and a QA/QC program put in place.

The team works in a two-tiered manner: The bulk of the inventory is handled by crews using decimeter-grade mapping with the Trimble GeoXH, with an external antenna on a pole and connected to the WSRN real-time network in VRS mode, this via a MiFi-type cellular modem. ArcPad is the field-mapping software on the rover, with a standard data dictionary developed for the project.

For features that require higher precisions (for the vertical component in particular), when that data may be needed for drainage computations and modeling, this is done by a fill-in crew using surveying-grade Trimble R6 rovers using the network. This phase is also overseen by a licensed surveyor. Solutions for maintaining real-time corrections in some of the “cellular challenged” areas of the city include a base radio and an Intuicom RTK Bridge.

Canopy is an ever-present challenge. The teams have found though that in nearly every instance, careful field procedures and keeping an eye on the data quality indicators and check shots are working well. The under-canopy features of units like the Geo6000 are expected to enable completion of the more challenging canopy-hell areas of the city avoided so far.

Cost of Avoidance

When faced with challenges like canopy—and weighing them with the nearly prohibitive costs of trying to do the same, huge amount of inventory effectively and rapidly with methods other than GNSS—people will always argue, “There are new tools coming; let’s wait.” In the case of the SWAMP program, there are certain permitting mandates that can’t wait, but also there is the ongoing cost of dealing with bad drainage situations. Annual costs of emergency fixes far outweigh the total costs of a comprehensive mapping project. As many are finding out in the world of asset inventory and mapping, success is not going to let a few trees get in the way.

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Bradley Bryan is a mapping specialist with Seattle Public Utilities who has been in engineering technology, CAD, mapping, GNSS, and GIS for over 20 years. Brad is currently developing field asset inventory procedures and solutions for the SWAMP team featured in this article.

Gavin Schrock
, PLS is a surveyor, technology writer, and operator of an RTN. He’s also on our editorial board.

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