Dynamic marriage of survey technologies

June 2012 » Features » ENGINEERED SOLUTIONS
Multiple-technology approach overcomes site challenges and provides 3D views for design, environmental mitigation, and public outreach.
Steve Mendenhall, P.L.S.
An eTrac remote-controlled survey vessel, operating in a shallow-water creek environment, incorporates sonar and a robotic total station.

Niles Canyon is a 7-1/2-mile creek tributary flowing into San Francisco Bay that used to have a vibrant fish habitat. The California Department of Transportation (Caltrans) wanted to increase the safety by widening the road through Niles Canyon, but was meeting opposition from the California Department of Fish and Game (DFG). DFG planned to re-introduce fish into the creek and was concerned that the road widening would interfere with its plans.

Caltrans' District 4 Environmental Division brought Psomas on board to become a member of the Survey Team and provide a solution that would give the Environmental Division a study tool and DFG what it needed to study the impacts of the road improvements on the riverbed. The Caltrans Niles Canyon project represents a dynamic marriage of surveying technologies that produced a wealth of data to be used for multiple purposes by public agencies. The project was not accomplished without first overcoming some significant challenges.

Marrying three technologies
Due to unique site conditions, surveying Niles Canyon required the merging of laser scanning technology with hydrographic and topographic technologies. Psomas teamed with eTrac Engineering, a firm specializing in combining surveying technologies. Hydrographic surveys were performed in bodies of water inaccessible along the 7-plus-mile stretch of the creek bed; laser scanning was used to collect creek bed data that was out of the water and covered with a variety of small to large boulders, and topographic terrestrial surveys were relied on for creek bed data in smoother exposed areas and shallow submerged areas the craft could not access. This multiple-technology approach provided the ability to view the site from any angle in a 3D environment. In addition, there will be minimal need to return to the site for additional data.

The wealth of survey data produced will be used by Caltrans to address design issues and DFG concerns, in addition to being a valuable public outreach tool in response to opposition to the project. Detailed information is available in a variety of deliverable formats to mine for multiple disciplines, including engineering, hydrology, surveying, facilities, public outreach, contracts/solicitations, marketing, environmental mitigation, construction, management, and quality control, among others.

The following significant challenges are part of what makes the project unique:

Inaccessible site – The creek was inaccessible by boat. Traditionally, multiple crews would be employed over several weeks to cover the large, complex survey area. Total station operators would operate standard terrestrial equipment and rod-carrying personnel would be in waders and weights in the rushing water. Line-of-sight operations would be difficult with total station work due to the extensively vegetated canyon/riparian environment.

A processed point cloud of the Niles Bridge project site was generated from the LiDAR system.

In the Niles Canyon project, specially designed equipment housing laser scanning, hydrography, and topography technologies was provided to survey the creek bed. The specially built small, lightweight, unmanned remote-controlled craft performed hydrographic data acquisition in difficult-to-access and hazardous areas that full-size craft could not provide. The shallow-draft vessel (less than 0.4 foot) was equipped with electric propulsion and protected propellers to minimize environmental noise and physical impacts on the ecosystem. On-board RTK GNSS (high-level accuracy GPS based on Russian and U.S. satellite systems) provided centimeter-level measurement in the canyon environment. A modular antenna mount accommodated 360-degree prism and tracking from a robotic total station when GPS was not available.

The vessel provided a number of benefits. It was low cost to deploy, necessitating only a two-person crew. Savings also were reflected in the low cost of running a vessel on electric charge as opposed to fuel, and low maintenance costs compared with large survey craft. And most importantly, use of the vessel improved crew safety by keeping personnel out of hazardous terrain and moving water.

Restricted GPS use – Another challenge was that narrow canyon conditions restricted the use of GPS technology. To address this, additional GPS control was established between Caltrans site benchmarks. GPS rover systems used GLONASS (Russian) and GPS (U.S.) satellite constellations for accurate positional data collection. Planning software was used to determine the best times of day in various areas of the canyon to perform data acquisition. Waterproof data collectors were used to display CAD layers to GPS operators to ensure they were properly oriented during data acquisition in the creek bed and surrounding areas. Real-time GPS quality information was displayed to the operator on the data collector. Minimum GPS satellite tolerance standards were applied on the data collectors to reject inaccurate data.

Manipulating data files from different technologies – Merging and manipulating site data files from different technologies was a challenge that was addressed with a select array of software combinations and protocols. Four specialized hydrographic software packages were used for acquisition and processing of water-based surveying and three specialized laser scanning-specific software packages were used for acquisition and processing of LiDAR terrestrial-based surveying. Data was processed each day to provide quick quality control checks and to monitor project progress. A common output format was developed and a standardized dataset of the merged technologies was delivered.

The approach used on the Niles Canyon project has since been used on several projects. Multibeam sonar, coupled with mobile LiDAR, provides a full-coverage, comprehensive survey of bridge structures and canyon/riparian environments. Custom, remote-controlled craft employing Multibeam and RTK/motion sensors are coupled with mobile LiDAR acquisition to cost-effectively and safely provide dense and accurate point clouds for assessment of concrete-lined aqueducts and levees.

In summing up the outcome of the Niles Canyon project for Caltrans, Nelson Aguilar, PLS, district office chief for R/W Field Survey Services, said, "Our job is to create value for our transportation facility users and stakeholders. In short, we look for ways to provide our survey services/products cheaper, faster, and safer. I have found that the best way to accomplish this is to create a team that combines the best we have to offer with the best the private sector has to offer. The outcome, as demonstrated in the Niles Canyon project, has been one that benefits the public and the team."

Steve Mendenhall is a vice president with Psomas in the Land Surveying and Mapping Group and was the project manager for the Caltrans Niles Canyon project.


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